there is.. except during disconnection (the time when you want to do PV measurement), the PV -ve terminal will be way below the battery -ve tab, this means you need either dual rail opamp or difference opamp that can accept out of rail (robust) input. once you got |voltage| wrt battery -ve tab, you can just add to battery +ve voltage to get PV open circuit voltage, but this can be much involved such as sensing/tapping mosfet's gate to know its state etc. care also during the night when PV -ve terminal = battery +Ve terminal, your difference amp will supposedly output -ve output, you'll need dual rail anyway, unless maybe you want to do measurement wrt the common +ve terminal. i have such common +ve PPM cheap china unit, made a teardown, and OP's idea passed through my mind, but i think its not worth the effort, so i just let it run as the way it is. i would say they are "not so stupid" at collecting energy from PV, they are less than $10 cheap and are okaish if you dont want to be so particular about wasted energy, a lot better than connecting directly the PV to the battery. ymmv.