So who keeps telling you only residents need to get the certificado de empadronamiento? I know different regions, and in some cases different towns, have different rules but the padron should be entirely different from residency. Where I live, everyone is encouraged to sign on the padron, residence or not. In Comunidad Valencia, and I'm sure other places, you can't buy a car from a dealer without an up to date padron. It was one of the first things we were asked for. There are several people in my street with cars who are only here for holidays and each one of those got the padron certificate to buy them despite never having gone through the residency procedures.
To take out a HP agreement in this area you need a padron but not residency.
Pop along to your local town hall to find out. Ours at Rojales asks for a passport and a copy of the nota simple if you don't have a residency card and will print it out for you there and then at no charge.
I'm not saying every dealer will ask for one as I'm sure there are plenty who skirt around the rules as in all things here. Reputable dealers will ask for it though.