Always believed this myself but noticed on the Metrobus site that the gates are locked at 22.45. Maybe it is ok for weekday evenings if you are prepared to walk back? I sent a message to Metrobus about using the facility for City games but didn't get a reply!
Long Ashton Park & Ride, BS3 2HB
1500 parking spaces
Free parking and no need to display a ticket
Open from 5.30am Monday to Saturday, closed Sundays and public holidays
Closed and locked overnight at 22.45pm Monday to Saturday
Overnight parking is not permitted
If your vehicle is locked in, please call 0117 903 1550. Bristol City Council retains the right to charge for the call out service and the vehicle release fee is £25.