My solution is similar to Neale's. The UB1 board I use has a simple safety-relay circuit, so the first 10 or so inputs can be linked to close a relay. My driver's fault signals feed into this, alongside my VFD's fault signal and a signal from my e-stop circuit. The driver's enable line is linked to the relay output, so if one drive faults the drives remain powered but not enabled. Same happens if the VFD drops out, or I hit the e-Stop button (but that also has the effect of killing the power to the drivers directly).