I know of at least one manufacturer who makes a listed lamp string. This is listed under decorative luminare and is limited to 25W per foot, is custom made to your specs and has moulded on lampholders. It is rated up to 16A per string at up to 250VAC. I don't remember the manufacturer offhand, but I did see this type at Macaroni Grill last night. It is an all black lampholder with a 2 wire heavy duty 12AWG zipcord with no cage on the individual lamps. I'll try to find this string again.
