I left the UK at the end of August heading for India without actually having a carnet. The only reason was simply that i always leave things to the last minute and decided to sort the Carnet out a day or so before i left! Anyway, the person i needed to speak to was not there! but i already had some forms so i ended up arranging it by post, phone and credit card in Slovakia. They then DHL'ed it to me.I gave them a forward booked hotel address in eastern Turkey so i had it for Iran. They could have just posted it but i didnt fancy that with the cost of a carnet and decided the sixty uk pounds delivery well worth it.
I would have thought that as long as you investigate the details first and get good contact details, they can fax forms to you anywhere, the UK RAC offered to do this with me, and then you can complete and post them back with credit cards details etc.
I would be surprised if you had a problem.
Some day so soon....