I'm running the numbers for enclosing a 2000 sq ft structure. I've got no end of 1 inch white pine T & G rendered out of the logs that gave us the timbers. I'd like to lay it diagonally across rafters on 36 inch centers, for aesthetic and stiffening effects. Is there any killer reason not to put a thick nailbase EPS panel (OSB on top side only) on top of that, and then apply roofing. What TYVEK, vapor barrier, roofing felt, etc. is recommended? Would more expensive polyisourethane sandwich panels be worth the cost?

I'd bite the bullet and go to structural (double OSB) panels, have seen way too many problems with nailbase and "curtainwall" (w/ drywall on one face). You only got one chance to make the roof stiff, not a place to skimp to save a grand - the nailbase gives you negligible (read NO) stiffness help. If strength is not important to you - use the nailbase. Any overhangs will require the DOSB. More chances to roll and flip scraps if you have all DOSB chunks (less waste, maybe).
I gave up on all the fancy "options" the panel folks like to push. DOSB everywhere for me.
In the field, you want the EPS foam - much easier to deal with. If shop cut, consider urethane. I find the urethane panels more inconsistent - thickness, can be challenging (especially 6").
Lots of opinions about using panels....

if we can believe the propaganda it appears that you can use the t8g to cover the rafters completely topped with 2" P2000 system and a metal roof. should give you r40 plus and the lightest roof around. cover the exposed ends of the insulation with one of the man made composite "lumber" boards and you avoid envirionmental deteriation over time.