That's actually a really good question! Most likely he hasn't discovered anything much on unsealed Metalminds, he seems very open to helping others, unlike the others from the original crew, like Kelsier or Harmony, each for various reasons. You also should take into account that spook got interested in Hemalurgy after the Catacandre and Marsh would probably be very welcome to being able to enhance his body. That could also mean that Marsh continued his tweaking as time went on because of spooks original acceptance to doing something like that. That could also be where various new discoveries about feruchemy came from, and maybe even some forwards on unsealed metalminds, given that Kelsier interacted with spook (See Mistborn: Secret History) and with this theory, Marsh too. I still don't think Marsh knows all that much on unsealed metalminds, based on earlier statements, but Hemalurgic testing could be very helpful in moving forward with either feruchemy or allomancy.
In the end, I think it unlikely that Marsh had any contact with unsealed metalminds, but do think it likely that he experimented on himself or someone else did.