Prompt for parameter for a header-level custom action?

Prompt for parameter for a header-level custom action?

I've got a header-level custom action that works on all the selected records in the view.  Is there any way to prompt once for a parameter that should be used for all the selected records? 

From what I've read in the forum, it sounds like I could use a pop-up stateless form for the input parameter but it would appear for each selected record.

Thanks.

- Craig