Sadly I've answered my own question . I posed the questionn to RYA and the answer is no...
Here is the results:
Dear Ethan I have received a reply from Richard Falk the RYA Training Manager and Chief Examiner and unfortunately, we are unable to accept your US 50 tonnes masters license
for the issue of an ICC. The RYA can only issue ICC's based on equivalency of UK issued qualifications. Therefore, you will be required to undertake a direct assessment with an RYA Recognised Training Centre. Please refer to our website under the link below and the attached ICC application form for further details. -------------