I don't believe that RC cars are even allowed at Valmont Bike Park. Even if there's no specific prohibition it just seems like other visitors would possibly give you a hard time (See: Girl OwNeD by RC TrUcK at BMX Track!). If you want unofficial jumps you may have better success with using the jumps in Gunbarrel (near Avery) or even what looks to be some social jumps on the north-east side of Stazio.

I tried for a long time to find places in Boulder to drive my fast RC cars. After too many experiences with flat, dirt roads or overgrown jumps or pebbles getting lodged in the drivetrains of my 4WD shortcourse trucks, I've accepted that the only way to really get a good experience with the offroad racing vehicles is to drive the 25 minutes to the Longmont Hobbytown track.

I recently started into scale RC trucks. My SCX10s don't require a lot of space and they make driving dry riverbeds, hiking trails and exposed rocks a LOT of fun. Currently, my favorite places to drive them are the woods behind my house, the nearby bridge-widening construction zone, and the riverbed at Four Mile Creek Trailhead.

Good luck and have fun.