Thanks to both for the help. I was able to get the ontimer working,
but did have to workaround 2 Excel quirks:

1) Application.CalculationInterruptKey = xlAnyKey seems to be changed
to xlNoKey (at least temporarily) after a large paste. I had to
include the Application.CalculationInterruptKey = xlAnyKey statement
inside my timer sub. I suspect this is the same "optimization" that
MS did that prevents interruption after paste in Automatic mode. I'm
hoping MS will be nice and create a registry flag to shut this off.

2) I lose the clipboard during a recalc. I had to check for
Application.CutCopyMode = 0 before executing the recalc.

I have a 3rd bug that I haven't been able to fix. The "end" mode
(when the END key is pressed) resets upon a recalc as well. Is there
a vba property that can read the "end" state? Googling "end" returns
way to many irrelevant results.

-MVL