When switching between multiple ASIO programs (like renoise and reaperin my case), it’d be way cool if renoise could automatically turn its audio engine off when in the background.
The way reaper does it is perfect. As soon as the program looses focus, its audio engine (and all media) goes offline, and the interface indicates it.
This also makes working with multiple instances of the program quite possible, even with single-client ASIO cards.
PS. As for interface, a simple checkbox in Prefs->Audio is all, and if you want to get fancy - grey out the whole main window in some fashion when offline.