These are my interpretations, (working for manufacturers of UL Listed products) and I am not sure if there are specific local guides that prohibit ANY modification of UL listed product - even if it is designed to be modified.
If something is just plain wrong, I would expect that it would specifically be prohibited in a recognized guide (i.e. NEC or local code, etc.).
Common sense would dictate that if I modified a product in a manner that it did not look like it was designed for, I probably would expect an AHJ to call for a field evaluation by a NRTL. An example would be to widen a receptacle blade slot to accept a polarized plug blade, or cut off the grounding pin off a UL listed extension cord.
Only so much can be spelled out in guides and specifications...the rest is left up to common sense and training (which some people may not have)...
Just look at all the violation pictures we have all seen of crazy things done to electrical equipment. I especially liked one that I saw of a handyman who epoxied a 200A switch lug to a bus bar because he lacked the proper bus hardware (EC&M). This is a field modification that lacked common sense and obvioulsy was a violation. There is also another one of a type T conduit body in which someone attached another fitting to the lid. This obviously also violates a few NEC articles.
-Larry