Hi Phill
That's a very specialized request.
As you know this feature is not available in the current mojoPortal WebStore. mojoPortal does have developer accessible code to Compress and Decompress zip files. For example mojoPortal will unzip a skin file and install a skin folder. However it does not currently have methods or over-rides for password protection to zip files.
Bottom line is that in order to get your required functionality, this is custom coding and must be written by a developer. An open source 3rd party dll may be needed. SharpZipLib comes to mind. DotNetZip is another but only works in medium trust.
Option 1. "Clone" the existing WebStore feature and add this functionality as a developer. You or someone else.
Option 2. Add this to the existing Webstore code (No Cloning - Bad Idea. Updates would be a nightmare someday)
Option 3. Hire Joe Audette to add this custom Webstore feature with turn on/off settings then share it with the community by having it added to the Core mojoPortal Code.
My manager has hired Joe (Option 3) once in the past when I got stuck and it was absolutely the correct decision.
Take Care
Rick Hubka