I think Spain qualifies through their low country possessions, which are in the HRE sometimes (when Spanish or owned by an HRE nation.) If you look at the religion screen you can see the countries included in the HRE election, although this differs from the mission HRE (such as No France in the HRE) which is listed on Huszics FAQ.
The electorate changes quite a bit, as early on, a lot of Italy is included.
If an electorate is conquered by a non-eligible HRE nation, it is removed from the HRE map, and vice versa, if the Polish HRE lands are conquered by a German state they get to be included in the HRE map.
You get to move your troops freely, get VPs, and your relationship with all HRE states gradually rises.