While working at that large national sign company that closed down back in January, we used an alternative listing agency for several sign designs we had. We went with Intertek/ETL in a time crunch as they had a shorter lead time for testing the designs (and were less expensive) than UL. This allowed listing a like family of signs (mostly incorporating gas pricers that didn't have UL themselves). Acceptance of the alternate mark was a concern, but to my knowledge we had no issues in the field of it being allowed. Intertek, and I imagine MET also, offered assistance should that case arise, to ensure acceptance. This is in their interest to educate and inform people.
Challenging with ETL was that we listed a design--say a family of similar signs with gas pricers or alternate lighting. We could add to the listing if we had a similar variation without additional testing. But it was not like being a UL shop where you could label anything that met the requirements. For example, to do something we had not listed yet channel letters would have involved submitting the design and possibly testing (temperature, water exclusion....). That's just the way they were set up. It was possible to get blanket listing by doing a large submittal, but we did not get that far into it. Does listing under MET allow the whole shop to produce listed products, per UL48, or is it more case-by-case? What does the startup/initial certification look like?