how do I force the reload of a subpanel action or the full subpanel?

In Cases I have the stock Email subpanel and a custom subpanel called Contributing Engineers which comes from a M-N relationship with Users.

On the Email Subpanel is a custom "Internal Compose"  action that opens the Email drawer with all the Contributing Engineers in the To address and some basic info from the case in the body of the email.

The action for Internal Compose uses an API to get the information it needs.
Of course when the page is loaded the button is generated with all the info. So far so good. It works well.

Here comes the problem:

When I add a Contributing Engineer, and then do an Internal Compose, the newly added Engineer is missing from the To (he wasn't there when the Internal Compose button was built by the API call).

... so the question is, can I force the system to reload the Action after I add the engineer?
or perhaps force a reload of the entire Email subpanel? 
or am I stuck with reloading the entire case?

thanks,
FrancescaS