If anyone builds this app, I'd like a slice of the pie, please :-)
I don't know how to do it without some kind of markup / document system (no morning coffee yet). I figure it wouldn't be that hard.
You could use a TDMS file(v1), which each channel is an item. When ran you give the program a password, which it checks against each channel, calculating the salted hash of your password. When it finds a matching hash it decrypts the document (saved as data within the channel).
This gives you a lot of plausibly defensibility because nobody understands TDMS file structure, not even people who work with them (it is an open standard, just nobody cares). And secondly, you decrypt the document and you get something out, even if that something isn't exactly correct.
I could likely push out a windows version by Saturday I guess if you don't mind it'd be using SHA-256 instead of [b/s]crypt for password checking. Maybe future updates to include some form of internal compression + some type of signing who last modified the document(s).
True crypt tells you how to set this up, but not do this automatically. Also it would require you selecting which volume, not just "insert password get document"
This would offer a higher degree of plausible-deniablity, and portability, by making it a file its not tied to one location. The structure of the file, and multiple hashes also grant plausable deniability why you can't just decrypt the entire file in one go and compare the 2 documents SHA hashes.
Would it stand up in court? No. It would help avoid less tech savvy people.
Final fize size of 746kb though made me feel a bit happy.
You can even take it a step further: full disk encryption, one key will give an innocent Windows install, and another key will give the diabolical plans.
However, information is only compressible to the extent of redundancy involved, so this can be spotted: compare the amount of encrypted data with the size of the innocent data.