I don't know a way to automate that in script. But you could do that with a macro. In the API (qvw in your installation folder), you have an example to format a field :
set fld=ActiveDocument.GetField("Amount")
set fprop = fld.GetProperties
fprop.NumberPresentation.Dec = asc(".")
fprop.NumberPresentation.Fmt = "# ##0.00"
fprop.NumberPresentation.nDec = 2
fprop.NumberPresentation.Thou = asc(" ")
fprop.NumberPresentation.Type = 11 'fixed decimal
fprop.NumberPresentation.UseThou = 1
fld.SetProperties fprop
With a "for" instruction, you could automate this action on a field list.