No hand slaps for me, the back of my forearm on my band hand gets slapped lightly when I shoot full butterfly frameless. As far as it concerns me, the strain of shooting a few dozen marbles on my finger and knuckle is offset by how much fun I have lol. I've had 4 uglies, but I've had dozens of beauties too.. I think if you're careful, focused and deliberate it can be safely done and I personally love it. Never shoot frameless with exhausted arms or hands, with alcohol in your system or with unmatched ammo to your bands/tubes.
Also as a personal side opinion, I'm not a fan of the frameless rings. Cool concept but there's other options I like better.
Be safe and shoot straight!