Mortgage for foreign prop.

No. They can't put a "mortgage" a property they can't legally repossess if you default on the loan. Any loan MUST be secured against a Belgian property.

The Belgian banks will only provide a mortgage for this if you already own property in Belgium. Try one of the Spanish banks established here in Brussels - I got a loan for a flat in Spain from them before I owned in Belgium.

Fortis bank gave a loan to friends to buy property in Bulgaria, with a very competitive interest rate, no Belgian security. I don't see a reason they should not give one for a property in Spain.
P.S. Those friends live in Belgium and have a stable income here.