Maybe on the sidecars the heavier engine load heats things up another notch.
The desert rallies comes to mind, with stages longer than the nominal Husaberg service interval. Day after day after day... Some extra cooling as well as oil volume would surely be very desireable for these racers. The newer LC4 with the spin-on filter keeps oil in the frame, as the old Yamaha XT500 did, an integral oil tank and cooler in a way, both are supposed to be very reliable. Considering that, a cooler might not be such a bad idea after all. Think of all the money you save on SEMs and the added reliability and then the longer oil change interval as well. Add the old style LC4 fine filter to the lines, it would likely add to reliability even further. Or if you can get an adapter for the spin-ons.
The KTM 58038005100 filter in the Husaberg engines, while better than nothing, is it anything else but a coarse filter anyway?
One poosible place to put a cooler:
http://www.kedo.com/ page 41 in the catalog. The straps holding the oil cooler are probably battery straps made of rubber.
I do not like the type of marketing seen in
http://www.joeracerproducts.com/ktm-oilcooler.php as it implies that the RFS engine is more or less useless unless you get this or that product. The stock RFS is a very fine engine, just consider its heritage just for one thing!