Regarding PCJP2003 shiny pichu eggs, we have solved the PID/IVs. Here is the algorithm, and keep in mind all results from rand() are 32bit.
seed = 0 -> 0xFFFF
randA() - unknown
randB() - unknown
pid = ((randC() >> 0x10) << 0x10)) | (randD() >> 0x10)
If force-shiny flag on (How this is determined we don't know)
shiny = (randE() >> 0x10) & 7
pid = (pid & 0xFFFF0000) | ((pidH ^ tid ^ sid) & ~0x7) | shiny)
IVs are generated normally on randF() and randG().
I think this is similar, if not exactly the same way as berry glitch zig, yet was released 8 months earlier.