When I open my pool (probably in the next 2 weeks) I'll slap something together and test it. In the meantime, a lightly modified Conshelf VI, some inner tube, and some yielded this:
View attachment 713832
It's ridiculous, but functional. I can confirm that this mask intakes to the upper portion, through mushroom valves into the lower portion, and exhausts "only" through 2 exhaust ports on either side of my chin. I put only in quotes, because the oral/nasal pocket seal is super crappy (alleviating Tridacna's concern about equalizing the upper half!). Purging the reg results in air blowing down the faceplate, and thence to the oral/nasal pocket.
It's extra stupid.... and I can't wait to open the pool and screw with it!