If all you have to do is slightly open up the ring on the bayonet, and you can easily do this with common hand tools (a file), then your arguement that it is not a bayonet mount will likely fall on deaf ears. You can expect that at trial, the prosecution expert will do exactly that and snap that knife right onto the rifle, right in front of the judge and jury.
If you just want the appearance of a bayonet lug on your postban, grind off the "ears" on the bayonet lug that the bayonet locks onto. Without the ears, you have no bayonet mount, just a useless stud that looks like one.