Is there a way by holding down a modifier key to change the output of all highlighted tracks at once? (like to a bus?)
The short answer is “No”. There are a couple of ways to do something similar, though: not exactly what you’re asking for, but maybe helpful.
- You can send the outputs of all tracks in a group to a new bus, by right-clicking the group header tab and choosing "Add New Subgroup Bus"
- You can make multiple connections in the "Audio Connections" window via click-&-drag: if you right-click the bus input and disable "Show individual ports" you can even connect or disconnect stereo (or multi-channel) tracks to a stereo bus in one go.
Hm, thanks. That’s not very handy. Am I the only one who’s missing this “feature”?
It has not really been mentioned very much in forums, on the mailing lists, on IRC, in the bug/feature tracker.
If you’re subgrouping to a bus, there are other reason to want things in a group, so adding a shortcut that ONLY does the routing is generally not as helpful.
Thanks. Not a shortcut but just a temporary modifier or just “all marked/highlighted at once”?
A modifier is necessarily a shortcut, since all keyboard and mouse actions are just shortcuts for things that can be done in other ways.
Select them all. Put them in a group. Subgroup. You’ll be glad you did, I think.
At least a workaround. 
Still more confusing for me.
Btw: Subgroups busses didn’t rename when you rename the group.