Short enough that we can have a codebox to make life I a bit easier.
InvOnSelect.txt (I assume that is Inventory on Select rather than invulnerable).
Code:
hook_0211F844_ov_0A:
STMFD SP!, {R0, R1, LR}
LDR R0, =0x04000130
LDRB R0, [R0]
LSR R0, #2
ANDS R0, #1
LDMNEFD SP!, {R0, R1, PC}
LDR R0, =0x0208B698
LDR R1, =0x009500DA
STR R1, [R0]
LDR R1, =0x09600D40
STR R1, [R0, #0x8]
LDR R1, =0x1
STR R1, [R0, #0x4]
STR R1, [R0, #0xC]
LDMFD SP!, {R0, R1, PC}
Anyway I am not sure what ended up going for this one as the NSMB hacking community did some fun things with assembly coding as far as getting it in there where just about everybody else (even pokemon) uses binary patches on the whole ROM, or on individual files and bundles the patch with something like ndstool to pull it apart and put it back together, possibly also a batch file to handle it all. There is good reason to do it that way (changes are far easy to both hacker and layman) but I would prefer to see both or a section in a readme. Anyway at best that is crying over milk possibly spilt a decade ago so enough of that.
A link that might get you started, or might not if drama happened (and it often does) and someone split things off and went their own way.
https://nsmbhd.net/thread/1281-how-asm-hacks-are-setup-tutorial/
Hopefully that will start to get you somewhere. Ultimately it should be a matter of "install this, put the file here, press compile, go" but I would not put any kind of real money on it.
You could probably port that to a direct patch as well even without all that, though it might need some skill rather than just assemble and overwrite.
hook_0211F844_ov_0A
Hooking is the process by which you add code to an existing program. ov_0A is probably overlay 0A (for code that is good to have but not always needed DS games set aside an area of memory to swap in and out as necessary), though I don't know if this requires something from the kit. 0211F844
https://web.archive.org/web/20200128125654/http://problemkaputt.de/gbatek.htm#armcpureference for various instructions
http://imrannazar.com/ARM-Opcode-Map for more.
http://www.coranac.com/tonc/text/asm.htm if you want a crash course in things.
Looking that 0x04000130 is the location of the "GBA" keys (the extra ones added for the DS are just after) so that looks fairly self contained, and for something that small (essentially having a button fire an in game function as opposed to when it normally happens) it is what I would expect. The LDR command is basically copy this data into the register, though square brackets mean use it as a memory address.
0x0208B698 is likely where the state of the box is held, not sure what the numbers starting with 9 are.
