Unearthed Arcana Varient Rogue - Sneak Attack for Fighter Feats

[Nonlethal Force]

I would allow a Martial Rogue Y to take Fighter X Feats.

Where a Y level Martial Rogue would have the same BAB as an X Fighter.

eg Weapon Specialisation: Fighter 4; Martial Rogue 6.

That's an interesting interpretation ... and although not specifically as per the RAW of UA ... UA is essentially a variant rulebook anyway so that seems a bit moot. I don't think your idea would particularily unbalance a game. However, I would say that they have to use one of their bonus "fighter" feats to take it, not a general feat that comes around every 3rd level.

I personally wouldn't use this, however, because it does still strip away the one thing that a fighter can truly own all to itself: Weapon Specialization (and on up the feat chain).