What I imagine is done... picture a bar end, one that goes inside the handlebar. Now imagine the barend being the same diameter as the rest of the bar, and being a few inches in length. Although I guess you could just machine down a piece of bar to fit and weld it on...
Naah, I'd leave those. The triangle bit - chainstay yoke - possibly, but I'd still not bother. Basically look at frames you can buy, see where they've been drilled and so, and copy that.
It's been done before. There are certain places you can do it, with no problem (or little problem) like the headtube, and the BB shell, and on gussets and possibly what your trials bike has instead of a seatpost. Other than that, I'd not bother.
It looks cool, I think it would be interesting to use... I'd be tempted to set the alternative brake as the front brake, and the conventional one as the back brake.