It is a substitution cypher in the newer files--still uses 0x100 byte array, so I'm guessing the old ones used that simple cypher system too just hid the array in the newer files. I got it running--posted over at coder's corner of rookies to see how the guys there found it to run.
I used Ida Pro to disassemble the boot and find the code (eventually) but the version of IDA that I have does not do the mips16 in the boot well--I can get something good enough, but the un-disassembled instructions can be a real pain. The main sw is all mips32, so that is not really an issue in trying to mod code there. I can give hints, IDC's for IDA (for boot or main) or just post some of my results if any are interested--even have source code for a simple mips32 disassembler that I found on the web and fixed up so that my lcc-win32 compiler liked it--not great disassembler, but....