My son had one of our KoAloha Concerts that was originally built for Raiatea Helm signed by Raiatea, and later by Nina Kealiiwahamana, both with magic markers. After a few months, I sprayed a few coats of McFadden's Nitro, paying particular attention to doing a tack coat first, then followed by the basic 3 coats over 3 hours. Of course I had to sand and buff it out. No problem.
KoAloha uses a catalyzed varnish finish, and the lacquer did not react with it - this would be your main concern with using lacquer: compatibility. This is why its always said to test on scrap. Normally, if lacquer is not compatible, you'll know right away, or at least the next day because it will eat through your current finish. If it cracks after time, then its really crappy lacquer, or its sprayed on too thick.
I'll have to disagree with ricdoug on the shellac issue - this would be one of the thinnest, some argue best (if French Polished) traditional finishes on an instrument. As I understand, its still the way Dave Means does it, and he's very good at it.
That being said, shellac will go over (or under) most anything. The only problem would be longevity, as it is a soft finish and requires white glove care if you want it to last.
-Aaron
