No no, I've checked this, it's translated to u:\c\mint\slb (confirmed by echo).
In this case you may be right, the SLB loading functions may be busted.
Interesting thing is while EDITOBJC.SLB was definitely loaded after forcing it to default path ($SYSDIR), gemma32.slb still doesn't work (or test.app for gemma still doesn't work). I know it's a long time but do you remember what requirements has to be passed to be able to load / use your gemma slb?
There are no other requirements, I think. Of course, the SLB files must be executables, so maybe this is a problem with FATFS recognizing the SLB extension an an executable. I have no way to trace that.
Pozdrawiam KMK