I just don't know if this is a real problem. The Phillies called up three starting pitchers last September (Suarez, De Los Santos and Anderson) but it wasn't until the 11th, and none of them made a start. Eickhoff made one. If you really want to look at a young starter (or even two) you can probably find a reason to put someone on the DL. Or just release someone, if you have a veteran 5th starter or free agent-to-be. Since you can't make trades in August I'm sure that will actually happen, both before and after 8/31.
Similarly, Walding got all of three ABs in September. Plouffe couldn't even get added back to the 40. Cozens saw a fair amount of action but with the new rule they could have just put Quinn or Williams on the DL. If you want to bring up a Halseley and really let him play you can. And if you just want to lengthen your bench you can. Can do one of each, even.
No reason to add relievers at all, teams will have already used far more than 8 over the course of the season (and they'll all be playoff-eligible) but no reason to have 9 (but if you want 9, you'll just have to value that more than a position player). And these days nobody waits until September to add a K-Rod (just look at Seranthony, and that will probably be Edgar Garcia this year too).
In 2017 the only call-ups were Crawford, Perkins and Jake Thompson. That seems exactly right to me. A prospect, a bench player and a pitcher (who was used as a reliever in this case).