Depends on the agent, I'm sure.
My MS was requested by an editor at a major house before it ever got to my agent; when I reminded my agent of this during the submission process, she didn't care for a variety of reasons. She seemed to have her own plan and her own people to submit to -- and it worked.
Another agent, who eventually rejected my MS, told me, "You can get in touch with the publisher who requested the manuscript on your own if I decide to represent you."
So I guess it just comes down to what the submission plan is. I'd definitely mention the house and your reasons for submitting there. No harm in that, I wouldn't think.