I don't know that there is a "rule" other than altering listed equipment.
If I may say so, I find the question to be a bit humorous seeing as you work work a gear manufacturer. Over the years the prohibiting factor I have most encountered is that the gear manufacturer will not allow it due to the listing.
(No disrespect intended)
Wow! i'm surprised there is no rule for that. It seems odd that i can bolt a machanical lug directly to one of the bolts, say a four bolt connection, to a bus bar connection. I thought that there was some rule stating that sort of like you cannot bolt a lug to a connection with an insulator.
Just curious, why would you think it would be altering my own equipment? We are a U.L. 891 shop. We can alter anybody's equipment as long as we do it in our shop.