Another possibility is reverse engineering sparsebundles and making them cross-platform. Some work on this has already been done by others.
That's of course a nice approach, too.
Re: 1PW, that whole world is quite different from what Espionage does.
I know, just wanted to point out that they found a really nice and clever way to address this particular problem. Having the whole logic and crypto stuff which is needed to access the data "anytime, anywhere" implemented in JavaScript is plain cool. I use this option rarely, but if I need to, I am extremely glad it's there. But I just brought this up as an inspiring example, it is clear to me that you cannot easily go the same route as what you are doing is very different.
FileVault, in my opinion, is somewhat lacking.
Yes, but I wasn't intending to claim that Espionage is no longer needed now that FileVault is there, on the contrary actually.
My point is this: Before FileVault 2, everybody travelling with a laptop was basically forced to use some third party encryption solution such as Espionage, since everybody has private data which they need to protect e.g. in case their device is stolen. A solution catering for this broad user base and use case must be easy to understand and use, which Espionage is and was IMO (yes, I see from the forums that there still are people that have problems understanding it, but that's partly a problem of educating them well enough).
Now that FileVault 2 is there, this base is covered,¹ so I assume people will turn to Espionage to "go some extra mile", i.e. they don't just strive for privacy, but they want safe and secure storage of important data. Data which they can neither afford to lose access to nor afford for it to become disclosed. The "no disclosure" part might be handled by Espionage quite well already, but do I see room for improvement on the "loss of access" front.

¹) Heck, the great thing about FileVault 2 on my MacBooks plus my multiple, distributed, and encrypted Time Machine backup USB drives is that I don't need to worry losing any of them anymore. Every single piece of hardware is disposable and the data is safe against failure, loss and theft. If something breaks or gets lost, I buy it anew. Espionage exists to solve a very different problem.