For the purposes of the activities included in RD 750/2014, can continued airworthiness be maintained in national centres approved by AESA?

Yes, but only for aircraft included in TAE.AER.GEN.300.  
For the rest of the aircraft included in TAE.AER.GEN.100 and TAE.AER.GEN.200, maintenance must be performed in organizations with EASA certificate (Part 145 or Part CAO).