Above BP is this hike a bike, or actually climbable by mortals? I'm assuming this spits you out across from that short trail that doesn't exist that cuts across the 6th?
Young mortals can ride 90%. For older mortals it's a 22 granny 28/32/36 rear cog grind. Really old mortals will need to rest against a tree every so often. IMO though it's secluded, mostly rideable and beats the Mtn Hwy road hands down. We've been riding up Fromme this way this since the early 90's and with the Ascent trail going in last year and the work that is currently being done on the second half of Ascent above the BP it will eventually become a much more popular way of getting up Fromme.
Personally I'd not recommend starting climbing at Dempsey/Braemar/Pennzoil access areas. The Dempsey and St Mary's trails and up to the BP are where you run into the most hikers and dog walkers so it's best if you can avoid those areas.