How effective is steaming a turkey tenderloin the size of a brisket to make it tender and juicy?
I'm considering this because turkey has a tendency to dry out as it's cooked.
Steaming generally doesn't make meat juicier -- it is just as easy to dry out a piece of meat with wet heat as it is with dry heat, if not easier.
There are two things that you can do to make your turkey juicier. The first is a brine, which Elendil suggests above. The second is to make sure you aren't overcooking. I'd suggest using a probe thermometer and pulling the tenderloin when it is at around 160F and letting it coast to your target temperature of 165F.
Brining is a better solution as it gives you juicy meat and extra flavour. Steaming would result in rather a bland taste.
Brining is essentially marinading the meat in a saltwater solution (usually with some extra flavourings like peppercorns etc) overnight. You then roast the meat as normal. Super juicy, super tasty results. There are lots of guides online.