Go over to BURP(http://burp.renderfarming.net/
) and start a dialog with Janus, he might be able to help a good bit here. He made a huge attempt to take BURP as a pay to play project and just couldn't make it happen.
Even with super encryption, the code/engine on the local users machine would need some means to decrypt it to process the work. The minute that is in place, you're running the risk of someone opening it up. Some of your examples could have the data-subset which would help a good bit, but others will require the "whole picture" to do any work at all. Be it a memory scrape, HDD IO watching or various other means, it would not be hard at all to gain access to unencrypted versions of these.
Also, 90% of users who are running a system like this actually want to know what they are doing. They want the little screensavers showing them proteins folding, or looking at stars etc. The users also care about HUGE metrics on the various tasks they are doing. Go checkout some of the BOINC projects like Milkyway@Home, Einstein@Home, Folding@Home. in BURP's example, you can actually see realtime images in the gallery of what you're currently rendering.
I'd love for you to find a means to make it happen, it just seems fairly technically impossible given what you're attempting to do.
I wish you the best of luck here.
Shameless plug here, but those of you with spare CPU cycles, go join Enigma@Home and help read Hitlers mail from WW2....