During the most recent fact to face meeting the topic of connecting plugs/slots on firstboot came up. The feedback we got was that its an important feature.
As a strawman here is a proposal how to do it. In the gadget.yaml we add a new
connect: - <snap-id>:<plug> <snap-id>:<slot> ...
For the implementations we have some options. The most simple one would be a task that runs as part of firstboot that simply does the connections. The downside of this is that the hooks will not be connected when they run and that subsequent install/remove will not take the connections into account (which may not be bad, depending on what we want).