All of my single stage presses are 7/8-14; don't own anything with the larger opening or a threaded bushing. That option is unnecessary for 95% of reloaders, I'd think.
One thing that I have thought about with this design- how hard (or easy) would it be to damage or bend the unit when mounted on a press- so that it would not function? Say someone leans down on the lube chamber, or other improper or careless methods/scenerios.
Simpler is better, all things considered, but maybe a little extra metal for reinforcement here and there wouldn't hurt, JMO.
I think you have the boolit sizing whipped, and besides needing a catch basin, some method of removing the lube piston to refill the chamber really needs addressed- the idea of using "string" to pull the it back out will not be sufficient once the lube grabs it and it gets all the way to the bottom. In your design, you have the ability to use the threaded rod (like Lyman & RCBS does) to raise or lower it. Are there any better ways?