Ribbon callback from built-in buttons to trigger additional code

[Andrew]

I want to provide one of the built-in buttons on a new ribbon and
leave it to function exactly as it does on the default ribbon, but I
want to know when the user clicks it. Specifically, I want to provide
a button which allows the user to draw a connector line but then
trigger some other code after the connector has been added. (I can't
insert the line using a macro because it would loose the placement
functionality). Does anyone know if this is possible?

Thanks a lot,
Andrew