UL 508A - Listed assembly in unlisted enclosure?
You are commingling the concept of compliance with the UL/ETL 508A standard with Listing (which is a service).  There are other organizations that also perform compliance testing to the UL/ETL 508A standard, although UL is among the best known.

It is not possible to obtain certification to 508A unless all components are Listed or Recognized or a certification/verification is obtained at the assembly level.  You can have your industrial control panel and custom enclosure verified and stickered in compliance with UL-508A as an assembly, provided that your custom enclosure meets the requirements of the standard.  

I have done this in the past to achieve Listing when a non-Listed/Recognized component or enclosure was required, although it is not a fast process.