Rgrodevant,
I did the same thing you did, for the same reason. I wanted a single encryption password for the entire volume that's much longer and more complex than the passwords used to authenticate and login the users. I wanted this password not to be linked to any user account. I was getting the same "Update Needed" message underneath the placeholder image used for an unknown user (or perhaps just used for a user image that is not defined).
Thru some Google-fu I can't clearly remember now, I happened across this page:
http://techjournal.318.com/general-technology/filevault-2-part-deux-enter-the-dr agon/
It didn't provide me with the exact solution, but it got me on the right track. It provided what I think is another way to accomplish what you and I accomplished, with the benefit of providing a disk icon and "Disk Password" text to replace the mystery photo icon and "Update Needed" text. However, I didn't want to decrypt and reencrypt the whole hard drive, and I still haven't tried his method, though I think it will work.
Where his article helped me was to turn me on to the wonderful world of "diskutil cs". Typing that command in a terminal window displayed a full list of usage commands, and among them was "changeVolumePassphrase". So here's what I did to ultimately solve my problem. In a terminal windows, logged on as an administrative user, I typed:
diskutil cs list
The resulting output displayed a heirarchy, from top to bottom, of the Logical Volume Group, the Physical Volume, the Logical Volume Family, and finally, the Logical Volume, which is what interests us: specifically, we want the UUID for the Logical Volume. For simplicity I selected the UUID in the terminal window and copied it to the clipboard. I then typed:
diskutil cs changeVolumePassphrase 9F203449-4173-49F9-B62E-C6276A29AC69
...pasting the UUID in from the clipboard. (Don't worry - that UUID above is fake, just in case it matters).
It prompted me for the old passphrase and the new passphrase. In my case I used the same one both times, after which it asked me to repeat the new passphrase as confirmation. After a few short moments of work and a few new lines of output, it confirmed by saying "Finished CoreStorage operation".
I immediately rebooted, and was greeted by a beautiful sight - the disk icon and "Disk Password" text above the password field. Mission accomplished!
I hope this helps you or anyone else hoping to implement whole disk encryption on a Mac using a single password (without a recovery key!) in a nice, polished-looking way vs. the unsightly (and possibly unsettling to users) Update Needed message on the EFI boot screen. In this way the disk encryption password can also be changed without having to decrypt and reencrypt the entire volume, and user passwords are not involved in the FileVault Encryption at all.