checked in objects in a sync pool gets cleaned up on GC. It used to clean the whole pool, but now I think it does half each GC cycle. If you want to say "objects checked in should live here forever and not free themselves unless I want them to" sync pool is not the tool for the job.

Well, it's a start. In fact the existing interface lends itself really well to a rolling window on-demand (de)allocator, especially with that New function you can supply.

Just pool could've mitigated your problem at least partially, is what I'm saying.