Once I'm happy with the position and function of the robot arm on the sideplates there, it's certainly possible to make a bolt on adapter that will fit a stock Bluerov2 frame. The arm location feels quite sturdy with the bottom plate attached.
The main difficulty with the robot arm other than the back mount, is going to be getting the servo wires through the penetrators/ Even the larger ones aren't big enough to pass the wires through without pulling the plastic ends off first, which won't be a big deal for me, but I can see some people might be concerned about. I plan to encapsulate the servo to arm connections outside the ROV in some sort of silicon seal. They should theoretically be fine even exposed directly to water though.
The servos themselves are going to be the waterproof ones blue robotics sells here, slightly modified. I will fill them with nonconductive mineral oil, replace the upper shaft bearing with a nylon bushing, and possibly re-pot the cable passthrough on the side of the servo if it looks like it is going to leak. The circuit board is already epoxy coated, the housing has rubber o-ring gaskets top and bottom, and the servo motor should work fine in mineral oil, so theoretically the whole thing should be pretty easy to get down do the 300ft deep range or deeper without too much problem.
I'll probably release the arm mount files open source at some point, and maybe offer a few kits through a reseller somewhere or something if there is interest. Not sure yet.