If you're not going to wash it, I'm not sure you need a medium at all. You can just use a wet paintbrush to move the ink around on the fabric. I've used a basic brush set & a couple blending sticks with my pencils & blocks and they work great. I just draw directly on my fabric & when I've got my first layer on there, I wet the fabric. I can then wet blend, or wait until it dries & then layer on another color and repeat as many times as necessary. I actually like doing that better as I have had more success getting the final colors/shading that I wanted vs starting with the medium at the get-go. You can also combine steps by simply wetting your Inktense pencil and then drawing (a bit like painting because the ink will bleed a bit) directly on the fabric.
If you plan to hang it near a sunny window, the sun may fade the ink, and if it gets very humid in your house, I would worry the ink might either run/bleed into the batting but otherwise you might be fine as long as you aren't trying to create an heirloom. But you could always just rinse your inked PFD before you quilt it. That's what I plan to do. I don't necessarily plan to wash my finished quilts. I don't know whether it's safe to iron the fabric with the medium on it and, of course, since PFD doesn't have any sizing in it, it tends to wrinkle quite easily.