A crossover relief won't work properly on a hydraulic cylinder circuit, it will only suit half of your needs. There a basically two needs for a relief valve on both lines of the thumb cylinder.
1) when you are operating the pedal to move thumb cylinder, say you're extending the cylinder until it reaches the end of cylinder stroke, cylinder bottoms out, but you continue holding pedal down. This will pop open the relief from line A (pressure line) and divert it to line B (return to tank through control valve). In this situation, it will work properly.
2) the situation where it won't work properly is you have the thumb cylinder extended, then roll the bucket into the thumb to grab something, but unintentionally roll bucket too hard into thumb, too much pressure for thumb cylinder to withstand. Relief pops open, main control valve ports are closed (you're not using the pedal), sends oil from piston side of cylinder to rod side of cylinder, but in this circumstance the cylinder can not retract because of difference in displacement between piston side and rod side of cylinder. In this case, there is a chance you could bend your thumb cylinder rod.
The only way you can properly protect a cylinder with an "in-line" relief arrangement is to provide a dedicated line to tank to dump oil.