[plt-scheme] blocking threads in mred

In mred, frame%'s display and continue, whereas dialog%'s stop when
(show #t)'d, and wait for the window to close.

When i was writing the guibuilder howto, I ran into a situation where
i had a frame and wanted it to act like a dialog.  (in particular i
wanted it to be able to return a value to the caller of the dialog). 
The solution I created for this was a solution where i used
continuations to create what i called a "Stopped Process Fork" (SPORK
:), to which matt imediately shut me down by blocking the
continuations ;) (aparently i wasn't supposed to be in there, as it's
not gaurenteed to work correctly).

Anyway, I have a new similar situation, but this time i was hopeing to
not block the menus & other objects in the system, while still
returning a value to the caller of the blocking object.  yield seems
to block everything including the menus.  Is there a traditional
solution to this?  maybe creating my own fork before a yield, then
jump the cloned thread over the yeild, whereas the primary thread
get's stuck on the yeild?

Corey