> Actually, I would like the opposite, to mark it unfinalized to resurrect the object more than once.

That is out of contract and goes against the guarantees on the GC and can (will) cause the finalizer of the object to be executed more than once. I don't think is a good idea to expose an API that allows breaking the guarantees that we give: every object is finalized once.

Also, taking into account that someone could mark externally objects as not finalized can make things more complicated for us to maintain in the collector.