I personally think that your idea to modify a twist-lock to accept Spyder style detents is a great idea. I'm not exactly sure how you'll hold the detents in the barrel when you take it out (tape? super glue?) but that's the only real problem I can see with it. Besides actually drilling the barrel in the right place. But it should be too hard if you have a drill press and measure very carefully. (A milling machine would be better, but a drill press could work.)
I say give it a try....but first I would practice on an old barrel that you don't care about. If you don't have one, I'm sure you can find one for real cheap somewhere since you wouldn't need to worry about condition.
As for your buddy with the Halo Too, he might find that he really doesn't need a Level 10. If he's just shooting mechanical semi with no RT effect, the Halo Too will feed faster than he can pull the trigger. Then as long as he has good detents that don't double-feed, he shouldn't have any problems. I ran my first classic mag for years and years without Level 10, just a Halo. Never had any chopping problems.
You can get lower feednecks too. If you trade in the powerfeed for a direct right feed and then chop it down so it doesn't stick up two inches, you can get some really small feednecks like the
PTP feedneck. Those are nearly as short as vertical feednecks, and you can still sight down the barrel.