Alright here goes:
1. Install Runescape normally
2. Install RuneMate on C: drive
3. Create a syslink from "D:\Users\YOURNAME" to "C:\Users\YOURNAME" for the following files and folder: "jagex_cl_oldschool_live.dat", "jagexappletviewer.preferences", "random.dat" and "jagexcache".
4. ???
5. Profit!