I think that there is no solution within the Intalio XFORMS / PIPA scenario.
If you want to keep using XFORMS, then you could hack the XFORM file to give the list a src. Look at standard XFORMS guides for this ... Intalio designer will destroy any hacking you do the next time the form is opened for editing.
Alternatively use another UI approach following the TEMPO documentation. The Intalio AJAX form (at Intalio 6.0) would allow you to set the dynamic list from a URL (front your MySQL db with a web service)

David

typo corrected AJAX replaced AXIS above