HV507 is an awesome little chip and it certainly has the potential to reduce the cost/complexity of future DropBot designs. The main limitation relative to photomos relays is that the outputs are DC, whereas DMF works best using bipolar AC.
The OpenDrop v3 uses some nifty level-shifting to generate a bipolar square wave which partially addresses this limitation. The max frequency is ~1kHz (limited by the ~1ms switching rate of the photomos switches used in the level-shifting circuit). In my experience, 1kHz is much better than DC, but 10kHz is better still (i.e., droplets move much more easily), which is why we use 10kHz as the default on the DropBot.
I have some ideas for how to get a HV507-based circuit switching a bipolar square wave up to 10kHz, but haven’t had the chance to test them out. Would be happy to discuss in more detail if anyone is interested in pursuing this line of development…