Possible to call someone up last possible second to have them for the playoffs and then send them down next year and call them up late enough in this second season that he doesn't get charged with a years service time? I know people don't advocate this strategy but he won't make any difference in my regular season but would be an impact player in the playoffs

What people do with a 25/55 dur/stam pitcher?

The objective is to win championships.  If you're in a position to do that, call him up.  I don't know how to work the timing next season, however.

If he's not on the 40 man roster, "last possible second" is too late.

But, yes, you can call a player up after the regular season and before playoffs start.     You can also send him back down if/when you're eliminated.

If he's good, I use 25/55 as a starter.   PC of 55/65.   He'll give you 4 innings, never get a win but will have a fine WHIP/ERA.  Hardball Dynasty – Fantasy Baseball Sim Games - Player Profile: Will Mahoney    Sucks that he's 23-52 for his career but I'll take 120 innings of 1.13/3.15 all day.   And I want to make sure I get as many of those innings as I can.   That can only be accomplished by starting.