I'm not sure I see your objection.
250.8 mentions screws as acceptable in two forms, and the panel itself comes with a green screw for bonding the case. Ground busses attach with screws. 250.8 also mentions 'assemblies.' So, where's the problem with the panel mounting screws?
Ah, maybe you're thinking of the 'no other purpose' regarding jumpers in junction boxes. OK, the top two mounting screws are for mounting; that third one is my bond. Voila!
Perhaps you would prefer to see a little green wire poking out of the panel, then attaching it to the column with ... you guessed it ... a screw?
Well, we could weld the things together ... except that would compromise the strength of the column. Incidentally ... aren't all those bits of structural steel held together by ... screws?
As far as bonding the service, that is an entirely different matter, and is more an instance of 'deliberately energizing' than 'likely to become energized.' We're using the structure - or more correctly, it's anchors - as a grounding electrode. (Which is why we use it for grounding transformers).
Let's face it; if we were expected to bond the structural steel, we would be allowed to use it as an equipment grounding conductor. Yet, we are forbidden to do so.
