Back on topic.
I've been thinking about this very topic for a long time now and don't have a good solution. I believe that all you should need is a PWM output to control the DPR valve. If you can control the duty cycle to the valve then you are in control of the current to the valve. And not very much power is required, the DPR valve is around 18-20Ohms and needs at most 150mA, around 0.5W.

I don't think it will work to just tie the injector outputs together to get a periodic signal, probably not fast enough and the DPR might actually react to the individual pulses of the injector.

So to do this the injector pulsewidth output from the MS would need to be converted to a duty cycle, 0-100%. The trick will be getting enough resolution, on my PL engine at WOT the DPR current was around 22mA and idle is around 5mA. So not much current is required, and AFR definitely responds to a 2mA change in DPR current.

The other thing to consider was the good point made above about overrun fuel cutoff. If the DPR current is not adequately reversed and there is some fuel dribble on overrun that could be a very bad thing to have such lean burn. If the injectors are completely closed off and the engine is just pumping air on overrun then no problem. But I can see how some small fuel flow could create big lean burn problems.