I've used Pirelli Scorpions on a Rockster before and they were good enough to get me through a day on Salisbury Plain in the drizzle and fine on tarmac.
The difficulty will be finding a tyre in that weird size the R1100R uses.
I'm no off-road expert but in my experience, as long as it's dry, then road tyres are ok for dirt roads, just bear in mind the limitations and ride accordingly. Unless you're Travis Pastrana obviously, in which case I'll shut up.