I could see it, but it would represent a significant amount of power creep in that, all other things being equal, most of the fighter/rogue skills are 2 point rather than 3 point like crit attack /back attack.
I'll admit, I completely forgot to look at the new build chart before making this suggestion. I was thinking about the old chart where only Disarm and Evade were 2 build or fewer.
So, in fairness to your point, I'll take a closer look at what I suggested.
First, Improved Slay / Assassinate, Wear Extra Armor, and Hearty don't matter. They aren't once per day abilities.
Slay / Assassinate - 1 per 30 bp - Should be fine given you can only buy one every 3 levels and you are actually giving up a darn good ability to use the magic item (better than a +1/+2 for ten minutes)
Fast Refit - If you are buying these just to power magic items, you are actually spending 5 build (4 as a dwarf), because each one requires a matching level of Blacksmith
Disarm - 1 per 15 bp - Because of the change to how this ability works (hit a shield, disarm the shield), I expect its utility just jumped significantly. I think this is a balanced trade-off
Waylay - This is the main potential abuse skill. For someone buying it purely to power magic items, there is no downside (no bp limit, no pre-reqs, etc.). For those not abusing it, I actually like that it gives an alternate use when waylay isn't an option (which is all too often... stupid undead)
Counteract - 1 per 10 bp - A minor potential abuse skill, but I think it has such value as a skill that I'd be dumbfounded if anyone bought it purely for magic item powering, rather than its main benefit
Intercept - 1 per 10 bp - Besides Waylay, the other major skill likely to be used to power magic items. Unlike Waylay, this one is limited by build total.
Unless I missed something, it looks like fighter and rogue each have one skill that could be used to take advantage of the system (rogue more than fighter). However, I'd be surprised if any fighter invested too heavily in Intercept without actually wanting the skill, because it basically becomes dead weight if you don't want the skill and ever find yourself without magic items (magic items do expire and can be lost). The rogue case is trickier because the skill doesn't become dead weight if you run out of magic items (except on undead weekends).
Hmm....
I'm leaning towards it being an acceptable avenue for abuse, since I find the abuse cases mostly unlikely, but I'm definitely mulling it over.
-MS