Thanks all for your answers and help.

I summarized what I've learned from the responses I've got so far and what I could find in others post in the net in case it could be useful in the future for someone with the same question.

- Most data seems to suggest that the chance to get through borders in Asia (Russia, Mongolia, stans) with a bike registered under someone else's name with out the owner being present is very unlikely.

- Atorney authorizations for driving someone else's vehicle from foreign countries may not be recognized or taken as enough proof of ownership.

- Crossing borders in Asia on rented motorcycles also seems to be either not possible or higly improbable.

- Having bike's insurance on your name or not is a different issue, this is about proof of ownership of the motorcycle.

- I've read about foreign people buying and registering bikes in Mongolia but they drove inside the country and sold them before flying out. They did not attempt to cross the border to Russia with them.

- An ADV member rode his bike from Austria to Magadan and then he took a flight back and had some other rider rode the bike back to Moscow. How he did it? I guess road Pólice is more flexible that the one at the Borders.

- Apparently in most EU countries for non-residents to get a vehicle registered on their name, they need to apply for a foreign register or tax number which is the same needed to allow them to rent a house, open a shop etc. Requirements vary but basically you need to justify why you need it, best way to justify the need is to have a contract for work there.

- Apparently non- residents can buy and register vehicles easier in Ireland than in other EU countries. There is a forum member in ADV that offer a service to help buy there.

Feel free to correct the info if so needed.

PD1: To the last response: yes I guess the dealer will probably sell me the bike, but they will have to provide the government office, that is the one that actually does the register, with a valid NIE number, which I don't have, now if they can do some "magic" there I don't know. For now I'm working to find out if I'm elegible to get a NIE number in Spain. thx!

PD2: It's funny, 20 years ago I bought a car in a used car market in Utrecht, Holland and registered it legally in just few hours... we traveled for 5 month through all Europe with it, got back and sold it legally again in the same place,...I guess the times has changed since then!