FWIW, on an N-reg I don't know of a way of getting the 337+AFMS approved. I have some contacts but nothing concrete. It's probably done by submitting the papers to a US FSDO via a contact in the USA
This is a problem for IFR approval in general for the N-reg (where you do need a 337) but not for PRNAV where there is an existing IFR approved GPS and it is PRNAV compliant. You do not have to have a Flight Manual Supplement, TGL10 permits a compliance statement. Therefore it is just a matter of documenting TGL10 compliance (iaw AC90-96A) and applying to the NY IFO for an LoA. I am not aware of this requirement for an EHSI - where did this come from?