This service manual for the 8502A may be newer than what you have. It contains the serial interface.
http://ftb.ko4bb.com/getsimple/index.php?id=manuals&dir=Fluke608 talks about the isolator. 608-9 covers the IC0-4.
Table 1-1 talks about the need for the isolator for remote operations.
2-8 has a warning about electrical shock and grounding. They take about reversing the input leads (input HI grounded and high voltage applied to LO). The don't mention the isolator.
4-71 warns again about common mode voltage when using the interconnect in place of the isolator.
I would imagine the 20V you mention would be with the interconnect installed but so far I have not found any such note. Even with the interconnect, I would not expect to connect the meter to a PC with a common ground, then attach a power source to the inputs with the LO also tied to the same common ground and having it short out some internal 20V supply. That's just scary. I would expect something like this to be on the cover page with large bold letters.
It's difficult for me to follow the schematics in this manual as they split them across multiple pages. On page 313 we can see the input transformers. On page 314 we can see the regulated supplies. Not how the isolated logic supply VCC is referenced to the analog -15 VA2. So from analog return to -15 then -15 to VCC and VCC to logic return, we have 20V. I wonder if this is what the note was talking about. Still this is all on the back side of the isolator.
I guess like anything, when you start probing around, it's best to understand what you are doing before diving in. I could see a beginner assuming all the grounds are common and damaging the meter or other equipment.