I've been out with fixed + front brake riders, but experienced, way lower ratio thatn the street too was considered de riguer. Never with a fixed/brakeless rider off road but it could be done though I don't imagine they could get going above the speed of a cross country runner. And they'd crash and burn, fast, even at low speed. Roots, etc etc were problems enough for -front brake- fixed riders due to frequent and hard to avoid pedal strike. No street cred awarded either, some deductions for being dumb enough to think they could match an equal rider on the right tool.