Sorry for not giving any updates in a while;
Yes the edited 0x1A command is at fault, which is odd because the command executes just fine when it's switching levels at the press start screen, but crashes when entering levels during gameplay. This seems to be the case for most emulators that I've tested it on.
This is my code for the new loader:
0x33910:
OR A0 R0 R0
LW A1 0x2C (SP) // source; currently holds rom address
LUI A2 0x801C
ADDIU A2 A2 0x1000 // destination; terrain texture bank const 0x801C1000
LW T0 0x30 (SP)
SUBU A3 T0 A1 // block size; destination end - destination start
JAL 0x803284B0 //(A0 = 0, A1 = src rom address, A2 = dest ram address, A3 = block size); function used for loading game sounds and mario's animations
NOP
Btw I'm doing a full rewrite, this time with my own mio0 decompressor function built in, so the rom extension process finishes immediately now

bin/src of this build here:
http://bin.smwcentral.net/u/25323/altextender2015-binsrc.zip
(I haven't bothered adding in all that extra stuff from the Oct5 release yet, this is a build I just whipped up quickly as a test for my function.)
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