To get the filter to apply, I added the following to the LinqDataSource:
<whereparameters><asp:ControlParameter name="PromoID" defaultvalue="0" type="Int32" controlid="promoIDs" /></whereparameters>
Where the ControlID is the id of the control on the master page.
This filters after a post back...which is a bit ugly, so there might be a better way.