Yes, get a QR plate designed for the lens and keep it in place.
I'd get a monopod that had a simple ball head and the same QR plate as your tripod.
First, consider if you want to switch to the much more universal Arca Swiss style QR plates. Your Manfroto head can be adapted to take them, mine is. Then, in the future only buy heads that accept Arca Swiss compatible plates. That opens you up to a much wider range of options rather than being tied to Manfroto.
+1 - I started with the Manfrotto RC2 system, and it was a weak link. There's a bit of play still when the plate is locked, which makes fine positioning on a tripod/ballhead a challenge sometimes.
I had a Manfrotto 694CX monopod with a 234 tilt head. The head is fine for lenses up to your 70-200/2.8 (I used it frequently with a gripped 7D 100-400). The 234 head comes in two flavors, one with an RC2 clamp, the other with just a 1/4"+3/8" reversible stud. I got the latter and put a Wimberley C-12 clamp on it for quick release with Arca-Swiss plates (a Wimberley P-20 on my 70-200/2.8, I now have a mix of Wimberley and RRS plates on all collared lenses).
Also, consider a Blackrapid strap for when you're carrying the lens around all day - much better than a neck strap. Attach the BR strap to the lens plate (if Manfrotto RC2, get the BR FastenR-T1 to replace the Manfrotto D-ring, if using an Arca plate, get a small clamp like the Kirk QRC-1" and connect the BR lug to that).