I don't know the AS/400, so this may be useless, but I wonder if a font creator would work.
I haven't used it for check signatures, but we have used it to put a logo on doucments in JDE UBEs.
The basic steps are to get a Font Creator package. Create a new true type font and assign a letter in the font to be the graphic of the signature (I recommend assigning it to both the lower and upper case). Install the font on the server and workstations. In JDE report design, create a constant with the letter you chose in the font creator and choose the new font. You will have to play with the font size and positioning in JDE to get the right graphic, but this works great for graphics in JDE.
Jer