Only one MUX of the '157 is used so that's the minute difference with 22V10 types. The normal 16V8 logic isn't very complicated (but it is secure too), I've determined (logically, not empirically) that it implements a 4 bit parallel-load SR and 2 bit counter for seamless reloading, I forget what the other two outputs are for... The PIC controlled (IIRC) MUX just selects between the PIC and drive data. A parallel interface is required since the PIC isn't nearly fast enough to serialize the data, it can barely keep up as is.
Decapping the PIC really isn't necessary, plus the decapping is only the start of that process, it needs to be doped and photographed, transcribed and reverified, then you need someone smart to figure out the decoding scheme from the rest of the layout. It's quite a process even when things go smoothly. Since basically all that's needed is the data stream and timing, the only real requirement is a clever person, their computer and some parallel I/O (parallel port, Arduino or whatever) to coax the data out of the modchip.