I may be off on this because I still am not sure what your "skid" is. My apolgies This my be cost prohibitive, but your may be able to get your "skids" listed if you mass produce (relative term) them. If the "skids" are unique from one another that you will not need to list them.
Either way they would need to be reviewed and sign off by a Electrical Engineer and the "skids" would have to assembled in an "engineer controlled environment" or by a licensed electrical contractor using listed conduits. Your local and state laws should address these matters.
If you are considering to "list" your skids, you are not limited to UL (no disrespect to UL) there are other Nationally Recognized Testing Laboratories (NRTL)
For more info to common questions on NRTL:
NRTL FAQ's NRTL website Underwriter Labratory listing process:
UL listing process 
