Verify Offline Bitcoin Bundle on Ubuntu
The offline bundle actually doesn't have a signature inside the deb file. You can see this by extracting the bundle as you did:

$ tar xvzf armory_0.92.3_offline_ubuntu_12.04-32.tar.gz


Now you can check the contents of the .deb file

$ cd OfflineBundle
$ ar vx armory_0.92.3_ubuntu-32bit.deb
x - debian-binary
x - control.tar.gz
x - data.tar.gz


If you download the armory_0.92.3_ubuntu-32.deb file directly, you can see that it's a little different

$ wget https://s3.amazonaws.com/bitcoinarmory-releases/armory_0.92.3_ubuntu-32bit.deb
$ ar vx armory_0.92.3_ubuntu-32bit.deb
x - debian-binary
x - control.tar.gz
x - data.tar.gz
x - _gpgbuilder
$ dpkg-sig --verify armory_0.92.3_ubuntu-32bit.deb 
Processing armory_0.92.3_ubuntu-32bit.deb...
GOODSIG _gpgbuilder 821F122936BDD565366AC36A4AB16AEA98832223 1412393663


dpkg-sig needs the _gpgbuilder file to check the signature against the contents of the package. The Offline package doesn't have this file while the direct download does.

I would suggest you grab the direct download and place that file in the OfflineBundle directory after verifying the signature if you're concerned about some sort of man-in-the-middle attack.