Is it a good thing to point this out in your header, or would it be considered a turn-off by people searching LinkedIn?

This goes to the question of whether LinkedIn is a sophisticated job board or a place to show your professional credentials. As with other posters, I feel LinkedIn should be used to "advertise" your professional expertise, no matter what it's in or how many fields of "expertise" you might have. In this case then yes, I think if possible (within the limited number of characters available) one should

However, it seems many corporations use LinkedIn like a job board; I don't know if that's true or not. If true, though, would a prospective hiring manager read your profile header and think, "This person isn't committed to working in (field)"?

The answer to this question is different depending upon how LinkedIn is used... and we just don't have enough information, at the granular level, to fully understand that. Or do we? If so, I haven't seen it; would someone kindly point me to that information if it exists?