Yesterday evening i managed to connect my external UMTS-Stick ( Huawei , free gift from Alice /O2 Provider in Germany ) via OTG Kabel
to my Nexus7.
Stock Rom, Android 4.1.1, rooted.
I used the app PPP Widget. ( xxx.draisberghof.de/android/pppwidget.html )
Everything worked like a charm.
I hardly found informations about Nexus7 working with this app, so i just wanted to spread the good news.
If someone needs information about the exact hardware version of this Huawei stick, i could post information later on.
As far as i tested, all applications got working internet connection.