I looked for the source of the maindec documentation I found before, unfortunately I wasn't able to find it. I thought I had saved a reference to it but I can't find it. I will keep trying, tho. It seemed like a great source.
One idea, and this is probably a stretch, but you might get some value out of this:
It is a write up on MAINDEC-15-D3RA-D. Yes, it is for the PDP-15, not the PDP-8. However I noticed that MAINDEC-xx-D3RA seemed to always be consistent in terms of being a DECTape exerciser diagnostic. The source code listing begins on page 18 and it looks remarkably PAL-8-ish. The PDP-15 was an 18 bit machine but apparently the basic instruction format was pretty similar to the -8.
In addition, you can find what reports to be a binary disassembly of maindec-08-d3ra here:
Of course it's not as useful as the original source code but this, in conjunction with the PDP-15 version, might give you enough to figure it all out.
Best of luck,