In the Solid Liquids Project I found that liqueurs sweetened with honey do not crystallize. (At least the honey-sweetened liqueurs that I tried.) I theorized on why and how we might over come this in this post.
However, in reading an unrelated book, I think I found the real reason these liqueurs are not crystallizing.
I was reading Bi-Rite Market's Eat Good Food: A Grocer's Guide to Shopping, Cooking & Creating Community Through Food and found the following in the section on honey:
Honey is a super-saturated solution, which means it has a tendency to crystallize (come out of solution) and turn solid over time. Because of this, most producers filter and pasteurize their honey to prevent crystallization and create a more uniform product.
Eureka! If it's one thing alcohol producers want, it is products that are consistent and don't spoil or separate in the bottle. My guess is liqueur producers who use honey use pasteurized honey, and that this is why liqueurs sweetened with honey have not crystallized in my experiments.
The Solid Liquids Project index is at this link.