With some thought it is possible to install e.g. the Bohm garbage collector when using old school objects.
If you've managed to write an fp memory manager with Boehm backend, then everything else would be using that memory manager and should work anywhere memory allocation is required. Not limited to old school objects or some other types.
Article 21646 by Barry Kelly on the delphi community forum.
I did one for KOL and another version for FPC if you are interested. But based on that code you are quite capable of doing that yourself in an hour or so.
It was discussed a couple of years ago with Florian.
Apart from performance issues (that could be/are overcome) Florian had several more objections, i believe because it also requires a paradigm shift and can not be used as is. You have to register objects for finalization, for one. But there is working code around, not only by Barry or me.