The format is specifically x64, if I wanted to write 32 bit assembler, I already have ML.EXE and a very mature support system for it. It certainly will work when fully converted for POASM in 64 bit but if JWASM or HJWASM is no longer 32 bit MASM compatible in its prototype format, then it would be no use to them.
This is how you do the standard Microsoft format.
wvsprintfA PROTO :QWORD,:QWORD,:VARARG
wvsprintfW PROTO :QWORD,:QWORD,:VARARG
IFNDEF __UNICODE__
wvsprintf equ <wvsprintfA>
ELSE
wvsprintf equ <wvsprintfW>
ENDIF