Ok...this was easier than I had realized. After some Googling, it looks like what I need here is the `POSIX path of` command. This converts a given path to a Unix style path, which is the type of path you would see/use in Terminal. So:

    POSIX path of input

Gives me a path like:

    /Users/Matthew/Documents/Programming/Apple\ Scripts/

It seems to take into account that you just need a `/` at the beginning of the path, rather than the hard drive name.