>> Note: currently there is a way to achieve that by pausing all the threads then selectively switching to a thread to make it current and setting the tracing function using the C-API (see: https://github.com/fabioz/PyDev.Debugger/blob/master/pydevd_attach_to_process/dll/attach.cpp#L1224), but I believe this is very hacky and not portable to other Python implementations.

> I'm not sure that I understand your need. Do you need a variant of PyEval_SetTrace() which accepts a tstate argument?

Yes