Even though i had a working prototype a little while ago, i think this idea is now pretty much complete. This is a recreation of Sharps CE-700E expansion chassis, a large box that houses an interface card, a backplane, a powersupply and a weirdly proprietary MFM harddiks controller.
Since my own usecase doesn't call for an MFM harddrive (or a extra powersupply), this project is just the interface card and backplane combined into a single card, which gives the Sharp PC7000 or PC7100 an ISA slot.
Due to architectual limitations this doesn't mean it can take any ISA card, most things don't work. But those chinese ISA to USB boards do work, and pretty well too.
But first, the hardware it's based on, the CE-700E expansion chassis:

My first working version was this one:

This week i received the new prototype boards, which i've named CE700minE, finished building the first board today and it's working.

The system boots from a gotek drive, loads the driver for the USB card from the gotek image and then uses the USB drive as any mass storage drive, it's also hot swapable. the USB stick i use is 2 gigs and for as far i can see it can be used fully.
The disktest results may not seem that impressive, but the real MFM harddrive in the original CE-700E is not that much faster in disktest.