Bear in mind that what the employee is lacking is appreciation. It's a basic need for almost everyone, and some people feel the lack of it more intensely than others. In view of that, here are a few ideas that could help, depending on your work situation:
1. Send around a group e-mail.
Send an email to the bosses saying something along the lines of,
I want to take a moment to recognize and express appreciation for the hard work and tireless effort of our fellow worker Mr. X. Without his aid, project ABC would have been totally impossible.
Detail how his work was not just important, but critical, for the project in discussion. Obviously, make sure your subordinate gets a carbon copy. :) You could even finish the email with something along the lines of:
Once again, I want to express my deep gratitude for his labours. If you have a moment to express your appreciation to him, please take the time to do so - as I have already detailed, the project would have been utterly impossible without his efforts.
2. Talk to other bosses in person.
Personal interaction is a major key. If you cannot interact personally with the other bosses, you will be severely crutched. Take the time to mention the work that your subordinate has done, being sure to give him / her all the praise, not taking any for yourself. Don't be afraid to say something along the lines of, "If you have a moment to express your appreciation to him / her, I know it would mean volumes to him / her.
3. Talk to your employee
Personal interaction is critical here. Be sure you don't just brush over the other bosses' actions; that will only put you in the same boat as the rest. But also, don't focus on their actions: this will only make his / her frame of mind more focused on it.
Rather, focus on the good work (s)he has done. If (s)he brings up the emails, look concerned; tell him / her it does concern you, but give the other bosses the benefit of the doubt. Put them in as good light as you can by suggesting possible valid motives, while not in the least discrediting the actions.
Just my two cents (which rounds down in Canada), but I hope it helps. :)