It's a 3-cell monitor using 2LEDs per cell, sat on the mouthpiece just in the corner of your field of view. Basically gives you advance warning of high or low PO2s. It's a 'Primary' HUD due to it's design (which I'm not going to go into, though if you're interested I can put you in touch with the mad inventor himself).
By tinkering, I meant look the unit over & see what'd be involved in plumbing it in, cell voltages etc
He just needs a bit more info on the technical side of things. Sound interesting?