Hi Chase,  The 712u is an iron hybrid and will use .355 taper tipped shafts.  Any of the shafts that we currently use for our irons will be available. 

That would be a heck of a custom club. Given Cathi's answer, you can still reshaft the 712U, the head will just have to be modified. Most clubmaking suppliers carry the drill bit that will enlarge the hosel to accept the .370 parallel DI shaft. This is what a touring pro would have done given your situation. DO NOT shave the tip to fit the tapered hosel. It can be done but is very painstaking and carries a much larger risk of shaft failure. Boring the hosel is considered a routine job to an experienced clubmaker.