I do not know about iron in the mesosphere. There are many other more active gasses, elements, and processes in the mesosphere which i can imagine to be more important in the effect of climate. However, when you append the global warming as questino, i must make you realize that everything affects global warming. Its a system-> the entire atmosphere sun, galactic rays, intergalactic rays, universe. Now, the question is how much of each and how they are coupled......and what we can do about it.
As for the effects on global warming, who knows? I dont lol...
The fact is though, that the number of meteors used to be a lot higher back in the early days (maybe we didn't even have a mesosphere then?) and has since tailed off to a more-or-less constant rate of bombardment. I guess that if the residence time for iron is low enough (considering it's so dense I don't think it could stay there so long?) some kind of equilibrium would be reached and the level would be pretty much constant.
Whatever, the fact is we have no influence or control over this so I doubt it's really at the top of anyones scientific agenda.