I found the answer to my admittedly naive question. My first attempt was temporarily "using" the NVMe fs and copying the resulting <start> section from /config/managed into the /config/managed file created when the USB fs is "used". This worked but was quite a kludgey process. Then quite by accident I discovered that "inspecting" a second filesystem makes it available for selection when setting up components.
With the NVMe available through "inspection" I was able to set up a file vault on the NVMe with my core system residing on USB. I was then able to set up a tinycore VM on the file vault fs.
I was quite happy to find that this is possible out of the box. I'd suggest elaborating on "inspection" in the documentation as this may be confusing for new users!