Eligibility in fantasy football in general is pretty subjective, though it has really far-ranging consequences for player valuation.
I'm not sure the best way to handle adding eligibility just yet, though having watched a little bit of the Packers, it is pretty clear that Ty Montgomery is actually taking a go at being a RB, which can't be ignored. Pryor is a bit different, as he (like Edelman could have in week 3) is only going to play QB because there are no other warm bodies to play QB for the trash fire that is the Cleveland Browns. So, in a completely subjective world, I'd be on-board with adding RB to Ty Montgomery's eligibility.
That being said, I'll play this more conservative this year, talk to some folks I know in the industry, and come up with a rule that is less at the whims of my subjectivity. I found https://help.yahoo.com/kb/SLN6318.html which sounds like what I'd like to have for Ottoneu Fantasy Football, but I am not sure how our data provider compares with Yahoo on this front.