In a project that's targeting Windows Phone 8.0, is there a way to conditionally use WP8.1 features while retaining compatibility with 8.0?
No you can not. If you need wp 8.1 features you need to target wp 8.1. According to adduplex 77.8% of windows phones are using wp 8.1 with 8.9 using wp8.
The only option to do this is to create a project that has a portable class library, wp8 and wp8.1 project in it. Put as much of code in the portable class library as possible. Publish both versions of the app. I would use something like MVVMLight for this