I have known about this before, and yes it can potentially remove the beautiful patina you have built up (the same with cooking a dish with a lot of liquid such as a thai green or red curry). However since steaming is a fairly quick method and you are not going to be doing it continually I believe it's OK to do. I do it with a couple of nice large 12" bamboo steamers, it's the easiest way to steam large amount of vegetables and other things.
If worst comes to the worst and some patina does flake off just re-season it. Or season it after you finish boiling/steaming anyway.