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I *think* external access to the tcl_lock would do it (it's
been a while).  There are a bunch of helper functions/macros
inside the Tkinter code for handling this situation;
exposing and documenting those would be ideal.

As far as error reporting, I don't think that suffices. 
(Again, it's been a while.)  The problem is that the
exception is returned to the C side, which must figure out
what to do with it.  Specifically, the Tcl event loop should
be stopped and the exception reported to whoever invoked
it... but there's no direct way to stop the Tcl event loop.
 (Fuzzy memories here.)