I think the real trick at some point is going to involve understanding how the IDE interface was done and implementing that with SDHC and a microcontroller. Ideally I think it'd be desirable for the result to look like a CF7+, and I don't yet know enough about how necessary/desirable that is.
I think that something like the NanoPEB might be OK... but we can do better. Say, a microcontroller that can read FAT32 SD cards, so that you can drag and drop PC99 or V9T9 disk images on the SD card on a PC and then put it right back on the TI. No HxC utility program to convert stuff to/from HFE files. No floppy controller needed for said HxC. No TI99DIR program needed to access the CF7/NanoPEB storage.
Throw in 32K and a serial or parallel port (or both), and we'd be good.
Ralph's cartridge based microcontroller is a good start... it can read/write FAT/32 SD cards and provides them to >6000, and can bank switch. We'd just have to interface that with a DSR now (in >4000 on the side), likely a larger microcontrolller (to handle more address lines), and we could then make it act as an actual disk.