mount encrypted sparseimage as home directory before login
So depending on the actual use case you are working with I may have an answer for your dilemma. If the location you are mounting to exists prior to actually mounting your disk, logging in before mounting the disk will create a default user filesystem with its very own login keychain. You can log in to a different account, mount the disk and save the password to the keychain, copy the password to the system keychain, and (after unmounting the disk) transfer the password to the unmounted user account login keychain. That item will exist prior to mounting so the disk can be automatically mounted on login. So you should be able to use that password with launch agents. Go into the password options and allow all applications to use it so you are not prompted for anything after it is in its final location.

If you cannot get launch agents to work, you can always write a short applescript app to refresh your finder to have the proper desktop to display and put it in the login items of the unmounted account so that it executes after the drive is mounted

delay 5
do shell script "killall Finder"


Put that into the /Users/Shared folder or something so it can be accessed before mounting and the mounting process does not interfere. 

So that "kind of" does not require user interaction at least beyond the process of logging on. I hope that helps!