Since you're already using a proxy for PA's why not just use AB's as your proxy? AB's are transparent and easily found - no need for complexities.
Because it doesn't work. AB+BB gets the majority of PAs taken care of. Outside of that there's just reached on error and HBP. HBP do vary by player, but only a few players have high enough totals for that to be impactful (and I'd include them if I could find the numbers, to be fair). ROE, as far as I know, isn't something that correlates with any particular skill, so they're even less significant (though, again, I'd include them if I could).
Anyway, why include BBs? First, unlike HBP, they are displayed, so I can, and it's more accurate. Second, unlike those other non-AB cases, they vary greatly player to player (and RBL team to RBL team), and have a dramatic impact on a player's and team's OBP. Third, specifically regarding the rule, I see no reason to punish a team that rostered, oh, I don't know, Joey Votto, for his tendency to walk. It's not as if he's less of a full-time player because of it. Sure, it would only come up if the team is skirting the line, but that line is going to be drawn somewhere, and if anyone ever does skirt it, they should only be punished for violating the spirit we're aiming at. Walks are legitimate instances of playing time and aren't the spirit we're aiming at.
It appears that this amendment might have legs so how about some numbers? I would argue for 100 on the low end and 200 on the high. We might have caught a few teams at the tail end of last season with the higher number, those who were stashing minor leaguers (Sluggers & Sauce come to mind).
Did you not see the numbers I already posted? 200 is definitely too high if you're talking ABs rather than AB+BB. Personally, I'd go no higher than 100 AB+BB, and would vote against the amendment if it went any higher.