That is exactly my point - you have to build multiple packages for the same app, even for the same version (app1_venv_a-1.0, app1_venv_b-1.0), if there is a need to have more than one instances of it running on the same machine.

Until emerge/apt-get/yum/etc add support for virtualenv, I will stick with .egg packages and be python-native rather than system-native.