Interestingly, I have an email from the CAA confirming that the IMC Rating is not restricted to a G-reg plane
If you want to fly an Outer Mongolian-registered aircraft, in Outer Mongolian airspace, then it is entirely up to the Outer Mongolian authorities to determine what their licensing requirements are. If they choose to accept a Double Dutch Ski-Float Rating, or a Russian Space Craft Rating, or a British IMC Rating, that is entirely their decision.
FFF
-------------
Disclaimer - for those of you with Double Dutch Ski-Float Ratings, I'm not certain that these are actually accepted in Outer Mongolia, and you should check with the Authority before you use it.