Depends on how you define the "must be OEM" line - but there's nothing in the rules that specifically prohibit them.
Personally, I take the approach that no EVO bike was originally fitted with USDs*, so they've clearly come from a non-Evo bike and therefore should not be allowed.
Then again, I'm a boring old stick in th e mud who thinks that CR480 forks and twin leader brakes shouldn't be allowed in Evo either.
*OK, OK: Honda Z50 - if you want to run Z50 forks on your race bike, then go right ahead.