If a Mico lock is what you have in mind, I'd have no problem with it for what you describe. We have supplemented a lot of tow trucks over the years with them. They are remarkably trouble free, very intuitive to use once you see the diagram of how they work, they have a bleeder if you need it, or bleed through if you don't need it so they install in about any orientation or location (which IS needed because the back part is a lot bigger than the handle part) the wearable parts come in a kit that takes two minutes to replace. Leak points are a non issue. It's a couple of pipe threads and a couple of double flares. Just like factory fittings, if the workmanship is good, the fitting, joint, and seal will outlast the vehicle without incident. The only weak link is the installation, of which you have the ultimate control to see that it is not a weak link.
They are NOT suitable from day one for long term parking. Which is fine, because neither is the Deuce's hydraulic system. As supplemental short term braking while the vehicle is attended, they will deliver nicely.
The flow is an issue. for the basic single lever single circuit valve, you've got to pick one axle to lock. The oriface size will not support two axles. You can lock the front, and two wheels will lock. You can lock either rear, and four wheels will lock for traction purposes, however the holding power remains that of two brakes, not four. Either option will be more than enough for what you describe. Mico does make larger stuff, but the price skyrockets as soon as you step away from the basic model.
The master cylinder was never intended for high pressure application. If you could introduce application pressure to the reservoir, it MAY be up to the task, but you are asking it to seal the full application pressure (as in a current "pushrod-boosted" automotive system), when it was designed and specified to hold only what you could build unassisted.
System air directly to the reservoir is not the solution to introduce high pressure to the master cylinder. It's probably too wet even with a dryer, and it's 125 pounds per square inch on a good day. The pressure you are looking for is probably closer to 2000 pounds per square inch, give or take a couple of hundred. Any locking that you do via this route will require manipulation on the air side of the brake booster to cause it to apply without a low pressure hydraulic input.
