Set custom DPI for different apps?
Maybe you should look into this ROM, however I cannot guarantee if there is an availability for the Nexus 7. 

To quote:


This rom lets you scale and project every app, every widget, even
systemcomponents individually.
Remember, android is modular, everything is an app: Lockscreen,
navigationbar, dialogs, popups, keyboards, widgets, and your regular
apps of course.
Apps have the capability to switch into various designs or layouts
according to the device they run on.


It is hybrid, meaning, that individual apps can change from tablet mode to phone layout mode on the fly. It is open sourced and is available on github.

Edit: 

Thanks to eldarerathis for supplying the linky to the N7 Version found on XDA.