It was not really difficult. I know where the booktype settings are stored in the drive. It's easy to locate the code that accesses these settings. For all firmwares with NEC style bitsetting commands, there are exactly two routines that deal with these values.
For bitsetting patches I only have to take the routine from a (somewhat) compatible drive, add it to the firmware I want to modify, change some offsets that are different between firmwares, adjust some memory locations, fix the checksum and that's it. Most of the time was needed for patching some subroutine calls, but I made a small script that calls a small external program and patches these calls.
I have to admit that adding bitsetting to this firmware took me less than half an hour :rolleyes:
BTW: From looking at some things in this firmware, I'm not that sure anymore, that the 3530 and 3540 can't be crossflashed. If someone flashed this firmware to his drive and knows how to execute a commandline program, please drop me a note. I want to get the safe mode firmware for this drive, but that one cannot be dumped with the original firmwares.