Another suggestion: The standard fairing mirrors have the mounting screws 40mm apart. This is a fairly common spacing amongst Japanese bikes . If you search for '40mm mirrors' on Ebay, then restrict it to 'motorcycle parts', you'll find plenty of them. If you get a pair like this:
(Suzuki GSXR range), they have a pivot near the bottom. This might allow the mirror to fold back enough to allow your bike through the gate. If not, you could replace the pivot screw with something quickly detachable, like a bicycle seatpost skewer:
When you wanted to go through the gate, flick the lever on the skewer to loosen it and undo the knurled nut, then remove the mirror.
Norman