Is there anything on the horizon in the 150gb+ range?
No. I think the iPod Classic 160 GB is the only DAP currently being produced over 150 GB, and as of yet there doesn't look to be any chance of a Rockbox port for it. (You can read about the porting effort for the iPod Classics, as well as the 3rd Gen Nano and the Touch
here.)
You can, however, upgrade an existing HD-based target with a larger capacity drive, though I *think* there is some sort of restriction to the capacity Rockbox can use and some sort of trick is required to be able to use the full capacity of the drive... (Can one of the devs or someone with more of a clue verify and/or expand on that please..?)