There are all sorts of USB-to-Foo interfaces, the vast majority of which implement whatever Foo is just barely good enough to say it works with a straight face. Like the USB-Serial adapters...most of them work ok at 9600,8,N, but trying 5-bit with 1.5 stop bits (something a 'real' PC serial port can be persuaded to do) and good luck.
So, yeah, I'm sure that USB-ISA adapter probably works with that old 3Com ethernet card or that Soundblaster card that 'definitely' sounds better than anything else, but a custom card with custom software and probably unknown tolerance to variations from 'real' ISA? Good luck with that.
Oh probably and even if you did find one that works "well" you'd have to buy a ton of them to ensure you have backups. I'm well aware of the issues. But without an alternative or a grant to update the hardware what do you do? At some point the availability of period hardware with ISA slots is going to be prohibitively expensive.
You can still buy new manufacture motherboards with ISA slots and modern processors. Intel still (or until recently) made ISA bridge chips for modern Southbridge chips. And it's something that a smallish FPGA could handle. There is a whole ecosystem of companies that recreate old-style motherboards for exactly this scenario.
My brother's company does IT stuff for the manufacturing sector. Very conservative. I've worked with a number of his customers to source exactly this sort of thing. Years ago they asked my thoughts on an Italian made cloth dying machine the size of a bus with a DOS PC controller and a minimally documented ISA controller. A mid 7-figure US$ setup; the company that built it is long gone. We found a shop that supplied new motherboards, modern (at the time...P3/P4 era I think) with ISA slots the vendor guaranteed it would be every bit as slow and weird as a PC/AT. Migrated everrything over, the company bought a stack of motherboard spares new, and I think they're still using them.
Now...you want a challenge? Migrating the proprietary, mostly undocumented software, all in Italian (we aren't native speakers...though my high school Latin occasionally helped) from an old ESDI disk to something, anything, else. That was much more interesting.
So, yeah, I'm sure that USB-ISA adapter probably works with that old 3Com ethernet card or that Soundblaster card that 'definitely' sounds better than anything else, but a custom card with custom software and probably unknown tolerance to variations from 'real' ISA? Good luck with that.