I created an Installer.app repository for ScummVM, and it's working!
Updated: If you're using the repository I just had posted that ended with "plist", please switch to this one. I got the new format I was trying for to work, and the old repository's been removed.
Here it is, for your convenience. It'd probably be smarter for you to host this, Vinterstum, since you'll be the one updating the packages and versions. The format I have this in is multiple files, so if you're interested just shoot me a PM with your E-mail address and I'd be happy to send them along. The default repository configuration that can be downloaded from iphone.nullriver.com is bugged in quite a few ways, as I found out the hard way tonight.
The repository: http://www.seriouslyslick.com/iphone/repository
To add this to Installer.app (version 3 or greater):
Run Installer on your iDevice, press "Sources" along the bottom, press the "Edit" button at the top-right corner, press the "Add" button in the top-left corner, type the URL for the repository (beware the keyboard autocorrection! It tries to capitalize the 'p' in 'iphone'), and press "OK".
Installer will update your sources, and ScummVM will become available in the "Games" section. The version number is the date of the release, in YYYYMMDD format.
Note: If you already have ScummVM installed and you want to use the repository so that updating is easier in the future, you must delete your current copy of ScummVM first.