This might be part of your answer:
Player Profile: Onelki Gregorius - Hardball Dynasty Baseball | WhatIfSports
Crappy IFA I've let hang around for 4 seasons on my Low A roster because I'm too lazy to cut him lol. He's mostly been inactive, there if I had injury problems, so he only has stats for Season 13 (despite being signed in 12, and being on the roster in 14/15). And he has 1 year of pro service. So to me it looks like if they don't play in a game, they don't accrue a year of service. Even if they're assigned to a roster. So I'm thinking even if you assign that guy, he won't get a year of service as long as he doesn't play a game.
What I'm not sure about is whether that would push back his development cycle or whether it starts the second you sign a guy. I think I've seen people ask it before, but don't think I've ever seen an answer unfortunately. If I had to guess, I would think it'd be more likely that you lose development by not getting a playoff bonus than you gain development by getting the extra year of the improvement window. That would make more logical sense to me, but it could easily go the other way.