How can I create a default value for a dynamic page?

I would like to create a default value for a required field based on the value of the the dynamic picklist.  (ie: When an activity is of a certain type, I want the subject line pre-populated.  For all other activity types, subject line can be blank.)

My challenges are:

1) A post default isn't working because anytime I try to save I get an error (due to the field being required).

2) A regular default value doesn't change based on the dynamic screen changing.

3) It's a system value so I can't remove the requirement for this particular dynamic screen.

Regards,

Lance