According to Joel Corry, a former agent who represented (among others) Jimmy Hitchcock, the reason Bell needs to suit up before game 10 is not because of service time but because of the calculation of the franchise tag salary.I've seen rumbles here and there that he doesn't need the service time to reach free agency but I don't truly understand it.
If I reading the article correctly, Bell already has five accrued seasons so it doesn't matter if he plays this season. (Article goes into what might happen if PIT tries to "toll" his season - something they can do with players who are under contract but don't play but since Bell isn't under contract, it doesn't matter.)
HOWEVER, Corry claims that if Bell doesn't play this season, and PIT franchises him again, the 144% would be off his 2016 salary of $12.12M. Note that even if he is correct, the math in his article doesn't work so maybe the overall point is wrong. Still, Corry seems pretty sure that he is correct so I thought I'd post it.