I have a question about conditional BP's

Say I have the following:
Call EAX
I can place conditional BP e.g. where EAX=='some value'

But say I want to identify where EAX is being set to 'some value', what options do I have?

Essentially I want to be able to place a conditional BP on a register, but that doesn't appear to be possible. Are there any plugins that provide this function?