I'd just seal it. Unless you're ingesting the lead paint, it's not an issue, and presumably you won't be eating the headboard. ;-D

I hope you'll post pictures! I've toyed with the idea of turning an old door (or something out of the ordinary) into a headboard myself. Would love to see how you did it.

I used an old pantry door from my 1926 house as a headboard.

Are you sure it's lead paint? You could test it with a lead paint kit if you're concerned.