I've thought about that, and in fact that's sort of the driving idea behind converting to EHB.@Steel City 06 Have you looked into an electric over hydraulic set-up that would be used for boat trailers? Typical electrically applied brakes, such as those on car haulers, travel trailers, etc. don't work on boat trailers, due to their use case (submerging into water). But...we all agree that surge brakes (those brakes that are applied by a master cylinder in a sliding coupler) suck pretty bad. They work..but they aren't great. So, there is an electrically controlled master, which uses some sort of prop valve to apply a variable force to a master cylinder. I'm probably oversimplifying, but you get the idea. You could probably work something out like that to accomplish what you're trying to accomplish. I didn't get more than a passing familiarity with the system, because I got out of boating and they were ridiculously expensive when the systems first came out.
However I'm not sure I could safely (and legally) plumb in a trailer brake actuator directly into a vehicle's brake lines in parallel with the existing master. I would have to get really fancy with check valves and solenoids to make it work, and if it's becoming that complex I'm not sure I could ever fully trust the brakes.
With vacuum brakes, hydroboost, or EHB, all of those will default to manual brakes should something fail. But plumb in a separate trailer brake actuator, and in at least one failure mode (such as an incorrect valve lineup) you wouldn't even have manual brakes as you'd simply be pumping fluid from one reservoir to another every time you push the pedal.
Even the Mico lock I've long wanted to add still allows for braking if it were to get stuck shut (the brakes would be stuck but you would still stop).