Title, essentially. I'd like to be able to configure an island structure to generate in other dimensions as well, rather than having them be entirely void outright.

what do you mean by this exactly? do you want the island management VIC has to happen in the end/nether? or you want custom world gen for the end/nether?

I want the island management from VIC to be able to happen in the nether and the end, yes.

Late reply, but this is doable by changing (in voidislandcontrol.cfg) baseDimension to the ID of the dimension you'd like to start in, and installing Perfect Spawn and creating a file called perfectspawn.json in your main directory (same place as mods folder) with the contents:
{ "general": { "initialSpawnDimension": same thing here } }