Concerning the brass plates:
"What were the 'brass plates' of the Book of Mormon? Following standard early modern English usage, the term brass in the Book of Mormon most likely has reference to various forms of the copper and tin alloy that we currently call bronze, rather than the alloy of copper and zinc now known as brass" (Hamblin, William J. "Sacred Writings on Bronze Plates in the Ancient Mediterranean" F.A.R.M.S. 1994, p. 2).
"Specific Hebrew examples of writing on metal plates are relatively limited in number, but clearly attest to the practice. There are five major examples:
1. The oldest example of Hebrew writing on metal is the engraved gold plate attached to the front of the turban of the high priest {at least 10C}. According to Exodus 28:36, Moses was ordered to "make a plate (tzitz) of pure gold, and engrave upon it as an engraved seal (khotem), 'Holy to Yahweh.'"
2. Excavations in the late 1970's uncovered First Temple period tombs at Ketef Hinnom, near Jerusalem. Among the artifacts discovered in this dig were two small silver plates dating to the seventh century B.C., containing the priestly benedictions found in Numbers 6:24-26 and representing "the earliest known fragments of the biblical text."
3. In 161 B.C. Judas Maccabaeus concluded a treaty with the Romans which "the Romans engraved on bronze tables and sent to Jerusalem for the Jews to keep there as a record" (1 Maccabees 8:22). Josephus' account states, however, that the Jews themselves engraved the document in bronze. In his analysis of this incident Jonathan Goldstein concludes that since there are no other known instances of Romans sending bronze treaties to their allies (as opposed to keeping copies of the treaties on bronze plates in Rome), Josephus' account is probably more accurate. Later, in 140 B.C., when Simon was proclaimed by the Jews as both high priest and prince, "they ordered that this text [of Simon's privileges and responsibilities] be drawn up on bronze tablets and set up in the precinct of the sanctuary [of the temple] in a conspicuous place and that copies of the tablets be placed in the treasury [of the temple] so as to be available for Simon and his sons." These examples clearly indicate that, following the common practice of most other cultures of the eastern Mediterranean, the Jews kept records of important historical documents on bronze plates in their temple.
4. The most well-known example of Hebrew writing on metal plates is the famous Copper Scroll (3C15) from Qumran {1C A.D.}, containing a list of hidden temple treasures. Although the origin and purpose of the Copper Scroll is widely debated, it is a clear example of an attempt to preserve an important sacred record by writing on copper/bronze (Heb. nechushah) plates and then hiding the document.
5. The Hebrew ritual magic and ascension text Sefer ha-Razim {late 3C A.D.} contains numerous references to writing on metal plates or amulets (Heb. tzitz).
In conclusion the evidence leaves no doubt that the Hebrews had a longstanding tradition dating at least to the First temple Period (i.e. well before 587 B.C.) Of writing sacred texts on metal plates for amulets, inscriptions, and literary documents" (Hamblin, 1994, pp. 3-7).
While the tradition of writing important documents on bronze plates may be accepted to explain the plates themselves, there is somewhat more to the brass plates in that they may have been written in something requiring the learning of the Egyptians to read them (Mosiah 1:4 "For it were not possible that our father, Lehi, could have remembered all these things, to have taught them to his children, except it were for the help of these plates; for he having been taught in the language of the Egyptians therefore he could read these engravings, and teach them to his children,...").
Dr. Hamblin cites an interesting example which may also support this idea:
"Byblos Syllabic texts. The earliest known example of mixing a Semitic language with modified Egyptian hieroglyphic characters is the Byblos Syllabic inscriptions (eighteenth century B.C.), from the city of Byblos on the Phoenician coast. This script is described as a "syllabary [that] is clearly inspired by the Egyptian hieroglyphic system, and in fact is the most important link known between the hieroglyphs and the Canaanite alphabet." Interestingly enough, most Byblos Syllabic texts were written on copper plates. Thus, it would not be unreasonable to describe the Byblos Syllabic texts as a Semitic language written on metal plates in "reformed Egyptian characters," which is precisely what the Book of Mormon describes" (Hamblin, "Reformed Egyptian" available from FARMS web site. See also Hamblin 1994 p. 8).
There are a couple of important points to note concerning the Byblos Syllabic texts. The first is that the date clearly precedes that of the Book of Mormon (importantly the early portions). Thus there is a precedence with sufficient time to both a "reformed Egyptian" and writing on plates.
The second issue is that the very fact that the brass plates were written in "reformed Egyptian" is the most likely reason why the rest of the plates would be written that way. The concept and tradition of mixing a syllabic character with a different underlying language would be part of the inheritance. The concept of recording on metal plates are also part of the cultural inheritance which crossed the sea. More than just the record of Lehi's ancestors, more than the preservation of the religious history (not to diminish that), the brass plates may have also formed the model for the recording of the Book of Mormon, both in the materials and in the conceptual script.