Right off the top of my head, I see some chiastic structure in the "immortal becomes mortal" (i.e. The Fall) vs. "mortal becomes immortal" (i.e. the Second Coming) dichotomy in the Bible, and that encompasses two entire Testaments. I'm sure you could find other chiastic elements if you cared to look.
But it'd be helpful for me if you could point out where this "megachiasmus" is so I can examine it for myself. Do you have chapter and verse?
Okay, I dug this out of an old correspondence, so we can start with it and I'll provide more details as requested:
To examine the complexity of BoM chiasmus, it stands to reason we should look at the most complex sample we can find. Unfortunately, it's over forty pages long, so I'll have to stick to the high points.
In 1st Nephi chapter one, at the end of the last verse, Nephi says he will show the reader "that the tender mercies of the Lord are all over" his chosen people. This frames his intent in writing this book, and lines things up for his thesis, as stated in 1 Nephi 3:7: "I will go and do the things that the Lord hath commanded for I know that he giveth no commandments unto the children of men, save he shall prepare a way for them that they may accomplish the thing which he commandeth them."
This thesis kicks off the quest for the Brass Plates of Laban, at the end of which we have a repetition of Nephi's thesis, this time voiced by his mother Saria at the end of verse 5:8: "...the Lord hath protected my sons, and delivered them out of the hands of Laban, and given them power whereby they could accomplish the thing which the Lord hath commanded them...."
These two statements form the endpoints of a chiasmus that stretches through the quest for the brass plates. I have discovered eight pairs in this chiasmus, which can be arranged thus:
a 3:7 Thesis 1--"...the things which the Lord hath commanded..."
-b 3:9 they take their tents and go traveling
--c 3:15 Bedoin oath "...as the Lord Liveth, and as we live..."
---d 3:19 Brass plates are valuable because they hold the language
----e 3:25 Laban takes away their property
-----f 3:28 Laman and Lemuel murmur
------g 3:29 An angel appears
-------h 3:31 "Laban...can command fifty..."
-------h' 4:1 "...Laban and his fifty..."
------g' 4:3 Nephi reminds them of the angel
-----f' 4:4 Laman and Lemuel continue to murmur
----e' 4:11 Nephi recalls Laban stole their property
---d' 4:15 Brass plates are valuable because they contain the Law
--c' 4:32 Bedoin oath "...as the Lord liveth, and as I live..."
-b' 4:38 they return to the tent of their father
a' 5:8 Thesis 2--"...the thing which the Lord hath commanded..."
Now we know that a and a' are major teaching points of any mnemonic chiasmus, and so it is here, but the real test is the center of the chiasmus, which is always the most important point. Here again this chiasmus passes the test: the center of the chiasmus is Nephi testifying that the Lord is "mightier than Laban and his fifty, or even than his tens of thousands".
Nephi's thesis returns in chapter 17, as the beginning of another chiasmus. Here are all the pairs I've found; please forgive me that I don't write out all the references this time:
A 17:3
-B 17:18
--C 17:19
---D 17:24
----E 17:30
-----F 17:33
------G 17:35 (beginning)
-------H 17:35 (end)
-------H' 17:38
------G' 17:40
-----F' 17:41
----E' 17:42
---D' 17:44
--C' 17:47
-B' 17:49
A' 17:50
As you might have already guessed, these two chiasm are mirror images of each other, thereby forming part of a larger chiasmus over the entire book. The end arrangement looks something like this:
a
-b
--c A
---d B
----e C,C'
-----fB'
----A'g
-------h
--------i
---------j
----------k
-----------l-D
------------m--E
-------------n---F
--------------o----G
---------------p-----H,H'
----------------q--G'
-----------------rF'
----------------E's
--------------D'---t
--------------------u
---------------------v--H
----------------------w----I
-----------------------x------J
------------------------y--------K
-------------------------z----------L
-------------------------z'---------L'
------------------------y'-------K'
-----------------------x'-----J'
----------------------w'---I'
---------------------v'-H'
--------------------u'
--------------D*---t'
----------------E*s'
-----------------r'F*
----------------q'--G*
---------------p'-----H*,H**
--------------o'----G**
-------------n'---F**
------------m'--E**
-----------l'-D**
----------k'
---------j'
--------i'
-------h'
------g'A*
-----f'-B*
----e'---C*,C**
---d'--B**
--c'-A**
-b'
a'