I've made some quite successful (given the rate they disappeared) gin & tonic cupcakes. I concentrated the gin but also reinforced the flavour.
I didn't document the process in quite enough detail to give a full recipe but I started with an orange cupcake recipe that used juice in the mixture. In place of the juice I used a reduction of gin, infused with added juniper berries and lime zest as it simmered*, strained (then diluted with tonic water and lime juice)**. The icing was a slightly wet buttercream with reduced tonic and lime as the liquid.
So the process will be far easier with spirits that have a dominant flavour that you can reinforce, as some will be lost to the reduction. In many cases you'd be talking about caramel and possibly vanilla, though some oak chips would make for an interesting experiment (possibly infuse for longer before reducing).
If you were starting with fortified wine or low-proof spirits and wanted to maintain the alcohol (or didn't mind), freeze-distilling would preserve volatile, alcohol-soluble flavourings. This wouldn't work over about 30% unless you have access to something colder than a normal freezer (dry ice, or perhaps trickery with freezing point depression using ice+calcium chloride - the more usual ice+table salt approach wouldn't go cold enough)
* Infusion, decoction, or something in between?
** Knowing that alcohol doesn't all boil off, especially in baking, I still wanted low alcohol, which is why I reduce the gin then added the tonic, rather than reducing/infusing the mixed G&T.