I have a US voucher (it is for a dollar amount) that I'm going to use for a mileage run and credit my UA account. Is there any way I can find, without the "hit or miss" approach, a listing of the US codeshares that fly on UA metal?

This may not be complete, but flights in the 6200s-6500s belong to UA. 7400s is UX (Trans States). 7500s is UX (Mesa). Some 5800s are UA, some are other airlines.

If you look at the PDF timetable on uswairways.com, it will list out all of the flights and places a triangle next to the codeshares... from there, it's fairly easy to figure out if UA flies that route or not based on city pairs.

I find that searching by schedule on the US site doesn't always show all their UA code shares. So I just find the UA flight # I want and call the US desk for their code share #.

If you search on ITA, you can specify that the flight has to occur on UA metal. (i.e. search for "lax :: o:ua" -> "bos :: o:ua") When the search results pop up, you can then isolate the US codeshares just by clicking on the carrier name.

After reading this again, why are you focused on UA metal? Is it because you are hoping for a day of flight upgrade to E+ because you have UA status (which I can't tell from your profile)? If it's not for that, you can fly any US flight and credit it to MP.

You may want to check the T&Cs of the US voucher. Some stipulate that you must fly US metal and not UA even tho it may be a codeshare.