Is it possible to install Ubuntu in parallel to Android on the Nexus 7 (dual-boot), or not?
If not, how hard would it be to add the feature? It should be just a matter of adding another partition and installing a boot manager, right?
Yes, it is possible. See this Video and follow the xda-developers thread linked there. In development.
There is no way to do it with the current image but it has been requested. At UDS it was discussed in several sessions and requested by people but there are no current plans to implement it. Here's the bug that was opened where you can track any work that occurs.