Like all the other big audio developers, Propellerhead does a great job at keeping their file format layout secret, so good luck finding someone to reverse-engineer it...

Not possible and this also applies to rewire.

From their page:

Requirements for becoming a REX2 developer

Propellerhead Software will issue licenses to companies in the audio or music industry interested in creating commercial REX2 applications (i.e. applications capable of reading REX2 files - writing REX2 files is not supported by the licenses).

There are no costs involved. Propellerhead Software will administer the authorization, copyrights, logos and trademarks. The company will also provide the licensee with technical support, free of charge.

Due to the legal aspects of the agreement you need to have a registered company in order to qualify as a REX2 developer. We do not allow private persons, schools, or companies in other types of businesses, to become licensors.

Developers are also assumed to be confident using the C language.

http://www.propellerheads.se/developer/index.cfm?fuseaction=get_article&article=rexinfo

So, let's form a non-profit consisting of the developers. Doesn't say anything about what KIND of company. ;) Besides, if they provide free support and the license doesn't cost anything... But of course, starting a non-prof probably does. Eh.. oh well. REX2 support would be freaking cool, ya gotta admit. ;) Thanks for checking for the info rakib. :)

I must stress out the part about creating commercial REX2 applications. Meaning, not open source and not free.

So no, it is not possible to get an license from propellerhead. But this doesn't mean that modplug can make their own type of sample/loop container.

But I would rather recommend supporting ACID, since it is just wave files with special tags.

Thanks for finding that, Rakib. I knew about the ReWire restrictions but I didn't know it's the same issue with REX. I guess this can be closed then.