Where does the code require the use of listed panels?
Well, we can start with 110.2, which says equipment "shall be acceptable only if approved."
Perhaps one might assert that 'approved' is not the same as 'listed.' In that instance, I would point you to 110.3(B), where it says listed equipment shall be used in accordance with the listing.
Please note that I never said anything 'negates' a listing. Substituting a field-made cover certainly puts it in the category of "we don't know," as explained by the White Book. That is, UL cannot vouch for anything that they have not examined. In effect, this means that UL will not agree that any part of the product, in any way, meets their standards- without examining it. Strictly speaking, you can't say the listing is 'violated,' because what is there just might be acceptable. Or, it might not. There's no way for UL to tell. UL's acceptance might depend on something as 'minor' as to where the original manufacturer stuck his lables. Will your replacement cover also list the various systems for which the panel was listed?
Ultimately, only UL can speak for UL. I happen to believe that I'm better at guessing what UL will say than most.
A field evaluation? Lots of luck. With rates starting in the thousands of dollars, that option is pretty much off the table. If you think the UL rep will just drop by the job site and say 'gee, that looks OK to me,' you have another think coming. Oh, no ... you get to fly the UL engineer there, rent his room, pay his hourly rate (with a daily minimum), maybe send pieces to the lab for further evaluation, etc. You want their opinion, you're going to pay for it.
Oddly enough, the NEC does not ever mention UL. They do, however, mention the AHJ. It's the AHJ's call. Now, if you can produce a standard that says 'make a cover this way,' and you did, chances are the AHJ will agree that what you have is fine. Likewise, if you show the AHJ a perfect copy, he'll likely approve.
To repeat myself: it's easier for all concerned if the AHJ never notices.
