Just a small update to (partly) answer my own question: apparently, the 3G modem in the Nexus 7 can be accessed via the cdc_acm Linux kernel driver. Great thanks to whoever activated that option in our hardware-adaptation kernel (I suspect Tuomas did this. :)).
To cut a long story short. After inserting my SIM card into the Nexus 7 (Note that I have disabled the PIN for this particular card.) a new “Mobile Broadband” device showed up in the network manager applet. According to the info there it is the device /dev/ttyACM0. Note that I have two ttyACMx devices here on the Nexus 7 3G version, 0 and 1. According to the modem-manager debug output the 3G modem is identified as “XMM6260”, which corresponds to the other sources that say this chip is built into the Nexus 7.
So far so good, however, there is the problem that the XMM6260 seems to use a (slightly?) different command set. Apparently ofono already has an according driver implementation for this chip but not modem-manager. I uploaded the debug output of modem-manager to pastebin: http://pastebin.com/d7wS98FU
Still, even though the modem does not work yet, it is great news that there is the kernel driver. All that is needed now to get internet via 3G working on Nexus 7 with PlasmaActive is either an adapted modem-manager or the integration of ofono.