400.8 says flexible cords can not be used as a substitute for fixed wiring. As soon as you hard wire the plug-strip I would consider it "fixed".
Replacing "molded" plugs does not change the UL Listing, however it may make the product unsuitable for performing its function. It is always up to the AHJ to accept or reject a product, the UL label just helps in the process.
From UL.com
What happens to the Listing if a UL-Listed product is modified in the field?
An authorized use of the UL Mark is the manufacturer's declaration that the product was originally manufactured in accordance with the applicable requirements when it was shipped from the factory. When a UL-Listed product is modified after it leaves the factory, UL has no way to determine if the product continues to comply with the safety requirements used to certify the product without investigating the modified product. UL can neither indicate that such modifications "void" the UL Mark, nor that the product continues to meet UL's safety requirements, unless the field modifications have been specifically investigated by UL. It is the responsibility of the Authority Having Jurisdiction (AHJ) to determine the acceptability of the modification or if the modifications are significant enough to require one of UL's Field Engineering Services staff members to evaluate the modified product. UL can assist the AHJ in making this determination.
If a party wishes UL to determine if the modifications made to a UL Listed product comply with UL requirements, the appropriate Field Engineering Service can be initiated to investigate the modifications. This investigation will only be conducted after UL consults with the AHJ to assure that UL's investigation addresses all areas of concern and meets all of the AHJ's needs.
