Damn. This server I used to be an admin on had Multiworlds.
They had several maps, which could be loaded (with /load <mapname>) and unloaded (with /unload <mapname>)
That's what I want for my server.
But since it's not coded in, I guess I'm outta luck.
I thought I would help
You need MCSharp
Its a server hosting program very similar to MinerCPP but more stable and supports multiple worlds in the exact way you described
The only downside is MCSharp lacks some of the features MinerCPP has