I also run and love frame bags. My first was an off-the-shelf Revelate bag semi-custom fit to my Surly ECR. It also did not go all the way to the bottom of the triangle and I was similarly frustrated by its failure to make use of the BEST space though I recognize that they dodged potential compatibility issues by avoiding the area where a front derailleur might hang or a tipple chainring might be running.
I don't run either of those so subsequent frame bags have been custom with a request not to shortcut the bottom of the triangle. I should note though, that my timing chain has worn a hole in one frame bag on my tandem so look out for similar issues if you go that route.
As a side note: My wife runs a frame bag on her Pugsley and the extra space afforded by the wider bottom bracket is similarly valuable but the wider bag is not so appreciated along the top tube.