I have drilled a few conduit bodies in some instances where I wanted to add support. Mostly due to the pulling that I would be doing later.
I have also drilled drain holes in conduit bodies.
We are required to 'arrange to drain'.
IMO a small hole in conduit body can do this.
Is it it A-OK with the manufactures?
I doubt they say so publicly.
But come on we drill enclosures all the time for mounting or entering.