Phu... found it:
$previewmessage in
newpost_preview.
I only want to change the Preview, not the WYSIWYG (because of what you said) or the original (BB).
bbcode_parse_complete is an interesting hook, but called for very different things...
This took some hours of debugging and understanding how vB works, but I hope not to forget what I learned

.
Thanks for your help!
Bruno