So why haven’t such an open source application materialized after all these years?

Well you'd need to fund the cost of a kernel driver signing certificate and get it through WHQL as well. That's just for a start. So this wouldn't be something you could build yourself.

Surely you’d just need to turn driver signing off

Back when I was a desktop Linux user, a very sizeable portion of the other people I knew that used it weren’t “software engineers”. They were ‘power users’. I don’t think that my experience was all that atypical.

The need is more personal than corporate.

There are corporate solutions (like the Paragon drivers), they just suck. Paragon tried merging this code into the Linux kernel, but the code was so bad and unmaintainable that it was rejected. The FOSS solution is using FUSE, but that requires a UNIX-like OS for easy porting. Unless Windows ditches the NT kernel altogether, I don't think you can expect native filesystem extensions beyond NFS support.

GNU philosophy is not about moneymaking or footing the bill. Sometimes a problem gets worse when you throw money at it.