Phillipe ended up turning to IBM support for help. Here's what they said:
In a remote outq (with an IP Adress), for printers like HP* for exemple, you must change the parameter with value 'XAIX' (in big caracters), like
Options de destination . . . . .DESTOPT('XAIX')
-- Michelle Davidson, editor, Search400.com
you are facing an argument not transmitted : the windows print server don’t understand (or take care) of the request.
Ask your printer vendor for a tested 400 configuration (probably a customised *WSCST)