I have no idea. Have you tried to just book it on a local on line site, just to check if it will accept a foreign credit card?

Another idea would be to ask a major travel agency such as AmEx if they can write the ticket for you; there will be a ticketing fee for this. In the past, AmEx PTS advertised that they routinely tried to save moey by writing a RT as two one ways whenever, for instance, the return was especially cheap starting from that point, so they seem to know and practice writing plane tickets from wherever if it's legal and cost effective.

I assume the plan is to abandon your RTW in AMS on the return rather than going to the Seychelles to complete the trip and then purchasing the return from there. I don't know whether this is allowed or whether it would be better to do a RT to/from the Seychelles for your positioning flights. Have you compared RT versus OW fares? Being forced to visit the Seychelles twice rather than only once might not be bad.