Working on shortening the front and and lowering rear suspension at present so the 17" experiment will have to wait a bit
Rare as hen's teeth http://www.ebay.de/itm/BMW-G-650-GS...791597?pt=Kraftrad_Felgen&hash=item4188034bad
I'm thinking of getting a Strada and doing the opposite as the OP, I want a 21" front wheel. Basically a Terra with ABS. I would also switch to the higher beak, if anyone would be interested in trading these parts. If I can talk my wife into the idea, the Strada wheels would be available. I'll post here if this becomes a reality.
Many have done this change, including myself. I found the Husky spoke wheels to be pretty reasonably priced from the dealer, but that was last year before the KTM takeover. Most of the wheels are interchangeable with BMW parts, so that should help your search. I've been quite happy with my homemade ABS Terra.