Good point about reaction from the wood. I don't think a sealant necessarily guarantees protection. This is not considered sufficient barrier for archival purposes with other artwork...it's a matter of degrees of obsession, but a 4-ply matboard is the recommended barrier. I believe some conservators even recommend a horizontal separation of 4 inches. Now this may be for framing a historical document in a national museum, for which no degradation is tolerable.
This is a different scenario, but a thin sealant (chemical layer) with another chemical layer (emulsion) directly in contact with it, and unpredictable behavior over time such as outgassing that may be 'dormant' stored in dark, but activated by sunlight...
If people do this and get satisfactory longevity, great. Just tell me to "be quiet".
My thoughts on a 'top coat' in lieu of glass would be (assuming compatibility with the emulsion and that such an item exists), perhaps a satin finish water-based urethane coating. The water-based urethanes I'm told do you have any color and do not yellow, while the solvent-based ones are already or become yellow.
Murray