It has been done (with varying degrees of success) and in light of the 450cc fourstroke restriction rule (2011/12 onwards in Dakar*
) it would seem (as you point out) that it could be an alternative.
Here is a 1990 perimeter framed KX 250
(rally modifications and a KDX 250 motor) as ridden in the 1990 Australian Safari by Allan Cunnynghame to a 5th place (all XR600's in front of him).
(sorry about the faded old print)
The minimum fuel distance of that event was 300km (50 km's MORE than todays Dakar spec.) While it is true that did not include the same (fuel consuming) soft sand dune running that Dakar typically includes... but the bike still required a capacity of around 35 liters to go the distance.
The little KX/KDX was in it's element in the tighter or more technical sections (especially when the fuel load got lower into the stages), compared to the larger capacity fourstroke bikes of the day. Talking to Allan about it afterwards, he said that the hardest part of riding the thing in a rally (as Neduro
alluded to earlier) was in fact; the long monotonous transports... the vibrations of the little 2 smoker were "mindnumbing" apparently.
This was 20 years ago... with some modern rally technology (lower COG tanks, cush rear hub etc.) maybe a 300cc two stroke is not such a silly alternative agains a 450cc fourstroke at Dakar*
*The up to 700cc single fourstrokes are still (at the moment) allowed to compete in the other FIM rally events.
, why not throw a larger tank, some navigation gear and a cush drive rear hub on an EXC 300
and head of to the next NRE
and the boys to do some "real world" field research...
ie. run it in rally conditions against other 450 and 690 cc rallye bikes?