I've known a few people that have had this bike and I have never heard a negative comment about them. I have seen ones with a tail rack and a hard luggage trunk. Where they were purchased I'm not sure, sorry. As far as the seat goes you may want to check Corbin, Sergeant or Mustang, if you have not already. You may be able to find an aftermarket subframe, I would at least replace the subframe bolts with some beefier hardware. For the luggage mounts, most places will make them with your specified dimensons from the bike.
As for the last part of your post. I would imagine that the reason for this bike not being as popular would be due to the parts avalibility, especially in remote corners of the globe. With the quality of the bike I couldn't forsee any major mechcanical problems arising. Other than the usual wear and tear of overland travel, fork seals, wheel bearings, brakes and rotors, sprockets and chain, etc.
The sites below may have some more useful information.
www.advrider.com
www.thumpertalk.com