One thing you could try if you can't get the registry to bend is to execute a custodianship/license agreement with the seller. You end up with a piece of paper that gives you all the rights of the owner, without being listed in WHOIS as the owner.
Take control of the DNS, the online account at the registry, the email address used for contact from the registry, etc. Use the domain as if you were the listed WHOIS owner.
Include in your custodianship agreement:
1. You get a perpetual, worldwide, sublicensable, royalty-free (etc.) license to use the domain.
2. You own the domain, but the seller is going to HOLD it for you in their name at no cost (other than you pay the annual registry fees).
3. The seller will immediately transfer the name to you or any party you name at no cost in the event that such a transfer is permitted by the registry (things change, and you want them to transfer to you when it's allowed.)
4. The seller is prohibited from using the domain or anything similar.
5. THe seller will cooperate fully with any required adminstrative duties related to the domain and the registry.
You'll need a competent IP attorney to draft this, someone who understands domains and also Spain.