"The biggest problem is that multibyte arithmetic has to be processed in LSB to MSB order, with the carry propagating from one byte to the next; but on the 6502 it's so much easier to count down than it is to count up..."

Not that I've tried this (haven't programmed in 6502 assembly in 30+ years now) but doesn't that suggest he should order the bytes in the opposite direction? (ie, if I'm understanding correctly, he's using little-endian now but his comments make me wonder if big-endian might be more efficient on 6502.)