You're right. The Hummer's, be it a H1, H2 or even an H3 are not currently approved in Europe. Simply because GM haven't went through the trouble to get them approved as of yet - looks to change as they were talking about actually importing them themselves (GM Austria I believe?).
That being said, there are lots of Hummers on the European roads and they are all approved by EU legislation. The thing is, they need to go through the SVA test (may be called something different in Finland) for the full vehicle check-up,
unless, you can provide them with a certificate of a previously approved vehicle of the same make, model and year - in which case they do a limited test only.
These certificates used to be sold for a quite lot of money across europe as it saved the private importer from doing all the work themself. The bottom line is, any vehicle can be made road-legal in Europe, even with a 1000hp engine if you so like, it just means they will need to take the full test.
As for only providing emission results, you should be alright as long as the HP isn't way overboard. Obviously, adding more HP needs better brakes etc etc etc which will all have a part to play in getting it approved in the first place.
Or you could just do what I'll be doing, drive it on US plates
(for a while atleast)