I believe it's a requirement of font forge that they're using.
It requires file paths, and it will be doing a large amount of reading. Easiest way of solving this is probably just to fake that your memory is a hard drive.
As other comments have stated, most operating systems do a pretty good job at caching this type of I/O anyway. It goes back to the idea of not optimizing until you know where the bottlenecks really are. A lot of time the underlying layers may take care of it for you.
If this program needed to operate from memory instead of a disk, why didn't the coder just... code it that way? Are they saying to use a RAM disk to ensure you don't encounter automatic paging out to disk by the OS?