Yes, Magic won't go 'negative'. So it's perfectly legal.
But Hobbes, the cheat here is that you basically 'lose' 2 points that you never had, then buy up as normal. If you were forced to first get up to Magic 4 with SAPs, then lose all 4 by Essence Loss, then pay karma for the 2 points, you'd be out 2 SAPs. Instead you drop to '-2' which is actually capped to 0, then go from 0 to 2.
Again, the GM can veto it. I always made clear I had veto rights with characters, and I in fact did veto some builds and provided input to tweak them. (I also vetoed Orgasm.)