Instead of the "Prisoner in the Vatican" compromise achieved during Italian unification, how about France offering the Comtat Venaissin back or another Catholic country offering refuge to Pius IX?
Venaissin: not gonna happen after the revolution (no, seriously).
Andorra and Lichtenstein, however, were OTL proposals, with Andorra being rather obviously easy.
Yeah, I got that impression.
And replacing one European microstate with another seems so obvious in hindsight - d'oh.