Jana Garnsworthy (asker) Jul 9, 2015:

Thank you both for your answers. In the end I opted for Procuration and Procurator, with a separate explanation of the problem to my client. "Officer holding a general power of attorney", as suggested by our German colleagues, was not really suitable for a contract, which uses that particular word a million times, nor for a title below the Prokurista's name at the end of a letter... Agreement of Proxy sounds somewhat limiting (but maybe that's just my, incorrect, assumption). Thank you both nevertheless ;)