[CREATE] How open/free do you consider PSD file format?

[Olivier BERTEN]

 I'm trying to write a Photoshop PAT codec for SwatchBooker but I'm
stuck between my will to preserve as much information as possible, and
the lack (or my lack of knowledge) of open file formats that could
preserve all characteristics of these patterns... Some features like
"multichannel" (multi-ink) or CMYK+alpha aren't available in any other
format I know... Patterns like these are rather unusual, to say the
less... but I'd like to support that format as completely as possible...

So the best solution, in term of content preservation, would be to save
these patterns as PSD but I'm not really comfortable with including that
possibility in my format. In the other hand, pretty much all free
graphic applications are able to deal with that format...

So I'd like to have other's opinions...

Olivier