You can force Poser to always load from a designated runtime by setting the libraryprefs.xml file to read-only. But you can't add runtimes while running Poser in that case.
Swticthing runtimes is tedious in P6 and P5: you always have to go up the library tree, up to the root, before you can change runtimes. P7 and Poser Pro handle this better, there's a dropdown that allows you to switch runtimes, no matter where you are in the library.
Most of my libraries were built when I was using P5, so most of my items reside in one big runtime (40 Gb), with a few extra runtimes for development projects.
Now that I use P7 and PPro, I make more use of external runtimes, but I haven't gotten around to the task of rebuilding the library structures.
I'm not looking forward to that task. Yes, I have all the installers, but I have shifted stuff around so much that the current runtime doesn't look like the installer content at all. Furthermore, I use a lot of freebies, many of them older ones, that had their geometries in the wrong place, textures right in the library folders, broken thumbnails. I fixed all those problems a long time ago, but I didn't fix the installers.... So I guess I'm sort of stuck to that big P5 runtime.

As for switching to an empty runtime, I'm considering writing a little resident application that monitors the libraryPrefs.xml file. If it has been changed and Poser is NOT running, it changes the "current library" to the designated startup runtime.
Nice little .NET project, I guess.

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