Assuming one has LR, Photoshop and Nik Plugins for Photoshop, is it possible to edit a RAW in Photoshop (from LR), apply Nik presets, and move back to LR, all in RAW format?
No, you cannot.
The Nik Lightroom plugins will require you export your files to TIFF. Really anything you leave Lightroom to do in Photoshop will require moving out of RAW and into a TIFF.
You will then end up with two files: a raw file with all the non-destructive edits done before you used the Nik plugin, and a TIFF containing the the cumulative edits done in raw, and then by the plugin. But you can't go back into LR and see any of the plugin alterations in the raw file, you'd have to work with the TIFF from then on.
The Nikon Capture NX2 software (also written by Nik) had better integration with Nik plugins - I remember certain things could be done with raw, but even there I think most filters required generating a TIFF file.