re Subharmonic Division

Chris: Before you refret your guitar to a subharmonic series, take a look at Erv Wilson's Diaphonic cycles at the Anaphoria site.
These combine subharmonic division with suboctave modularity at the 4th/5th or tritone-like intervals. Also there are Kathleen Schlesinger's harmoniai, so I'd try several candidate tunings on a synth or scala first.

There is also the Tanbur of Baghdad in Ellis's appendices to Helmholtz, whose fingerboard is divided into 40 parts and the strings tuned an 8/7 apart.

--john