There's a hacked solution at the Ubuntuforums. ~quack's solution is better, but more work.
edit: added the full text
I couldn't find anything on google about this, save compiling your own
'dummy package' (impossible without uninstalling the broken
package(s). unless you are lucky enough to already have the tools),
but searching my own hard drive gave me the answer eventually!
For anyone who'd like to know, here's how to "unbroken" a package in
synaptic / aptitude / apt-get, without uninstalling it.
First of all, make a note of the package you installed, and the
package(s) it depends upon that give it a broken status. Say I forced
an install of apples, which depends on the package obsolete which
isn't in the repository :
In a terminal type Code:
sudo gedit /var/lib/dkpg/status
search the file for apples until you find something like :
Package: apples Status: install ok installed Priority: optional
Section: libs Installed-Size: 316 Maintainer: Architecture: i386
Source: applesauce Version: 1.0.10-1 Depends: packageA, packageB,
obsolete Description: Apples on your desktop!
Remove obsolete from the Depends: row, save the file, and you're done.
Hope this helps some people out