Does an analog game have to be a full game or can it be a supplement for another game? 

I imagine a supplement would be fine but you have to keep in mind they will only read the first 4 pages of your rules. So if it's a supplement, you ideally want a one page RPG to go off and then you'd have 3 pages leftover for your project.

Given how much I like the idea I have (backgrounds for Troika! based on various 1928 works), I think I'll proceed regardless and also think about a four-page RPG to submit as a more orthodox entry, since multiple entries are allowed. 

Supplements are allowed but will be judged on their own and judges may not be familiar with the core game

Yeah, that much I expected, thank you!