Interesting question. And you would think the answer would be yes, but I don’t have any specific info. Good second point re pre-chewing conferring immunity. Same mechanism.
I wasn't saying anything about the exchange of premasticated food (or saliva) possibly conferring immunity. I'm saying simply that kissing is entirely explainable without recourse to immunity-based arguments (Occam's Razor). Perhaps, instead, it is derived from the practice, in prehistoric times, of people pre-chewing and directly transferring food to toothless tribe members (babies, oldsters) in order to nourish them (not to confer immunity, which the oldsters .- being oldsters - would have anyway, and which the babies - being breast-fed babies - would likewise have anyway).
Rather, transferring (often tough) premasticated food in this fashion (mouth-to-mouth) was probably commonplace, at least among intimate family members. It was probably pleasurable for both babies and mothers. In these tribes, kissing among adults was thus perhaps a "hold-over" or a "throw-back" to a behavior learnt in their childhood years. (However, it may be that, in primitive societies, kissing was much less prevalent than in modern societies.)
Regards,
Birds do it, bees do it, even monkeys in the trees do it.
Let’s do it. Let’s re-masticate then regorge our young’s food.
(oh, it sounds so romantic that way).