I’m jumping in a little late here, but you can definitely get reliable, drift-free sync between the 7D and external audio, as long as the external audio has reasonable clock accuracy. Like many others on this thread I highly recommend the Zoom H4n. We have measured it and it can hold sync to within one video frame for 100 minutes.
There is possibly another issue that is causing the drift you are seeing. Final Cut has a quirk that causes drift but can be avoided with a little care. You just have to make sure that the frame rate of the Easy Setup you are using matches the frame rate of your clips. If it doesn’t, change your Easy Setup, restart Final Cut and create your sequence again.
On the Canon 5D the drift is known as the 99.9% problem. On the 7D it is the 100.1% problem. The solution is the same. See this blog post and video tutorial for more information.
Bruce Sharpe
Singular Software