Well I had a go at this on my RPi. I altered libc.a to arrange that errstr() removes the error string filename decoration, but print ("%r") doesn't, and wrapped the open() call (as an example) to do its own error string decoration and hand that back to the kernel.

Unfortunately I then looked in the kernel and discovered that namec (which does the decorating) is called in about 50 places. If it stopped decorating error strings, there are potentially a lot of places that would notice, and I haven't chased them all down.  That being so, my enthusiasm for this idea is much dampened, and I think it's dipped below the worth-considering threshold. :(
I'll keep the code I wrote around in case it's ever useful. On the plus side, I learned some things about the libc.a build process. And the awesome mkfile in /sys/src/libc/9syscall.

I think it would still be worth fixing exportfs to strip off any decoration before sending out an error string via Rerror, though, as that would fix v9fs's problem and the nested mounts problem I was looking at earlier.

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