Does anyone know if it's OK to simply grind the locking surfaces off of the sides of the bayonet lug rather than cut the whole thing off when using a pre-ban barrel on a post ban rifle?

yes, it is.

Thanks, I should clarify a bit.

The bayonet only locks onto the two front teeth of the bayonet lug. The rearward protrusions can't hold the bayonet on.

Should I cut off the rearward ones also? There is no way for the bayonet to even go over them. It's actually a stop for when the bayonet locks onto the two front lugs.

If it is non functional, it is no longer a bayonet lug.

Just remove the front ones and it will be impossible to mount a bayo.  Shouldn't take more than a minute to do with a dremel.

Did that. Then touched it up with some cold blue paste. Looks nice and it still preserves that original AR sillouette. A CAR-15 just doesn't look right without the bayonet lug in the profile.

I wonder why more people don't do that instead of just cutting the whole thing off?

Put on a CavComp muzzlebrake. From a few feet away, the CAR-15 looks like a pre-ban. If AWB sunsets, I'll just Dremel out the welded pin, remove the CavComp, and install a Vortex flashhider.