It depends on the MILSIM event itself, but most do not accept hunting/Real Tree camo. Again, the accepted camo patterns for each side vary slightly but I do not recall ever attending a MILSIM event where hunting/Real Tree camo was accepted.
Wearing hunting camos to any game that is separated by green base and tan base camo doesn't really work. A couple of younger kids at a recent op wore Mossy Oak and everyone was shooting them because no one knew which side they were on.
A government that is big enough to give you everything you want, is strong enough to take everything you have. - Thomas Jefferson