with a 5/8 inch post this means the tool rest locking casting is NOT very strong... if you get a bad catch in hardwood out near the end of the rest you might fracture the casting... the longest tool rest available for mini lathes that is of good quality is the Robust 9" tool holder.... but Grapenuts idea is a good one... just be careful and get a good weld at the junction:
here is a link for the Robust tool from Craft Supplies in Utah..
Robust Comfort Tool Rest - Woodturners Catalog - Woodworking tools and supplies specializing in woodturning.