Yes, it can be done, but you'll have only mono sound on the 5D. Next week I'll start the post of a project where they've done it this way:
For the audio to get in the 5D, they used this mic adapter:
The sound girl send a mono mixdown signal, wireless to the 5d from her Cantar recorder.
For the timecode, they used a Clockit
Jammed every now en then with the recorder set on free run, and then hang the Clockit on the camerarig with timecode out in an audio signal.
When they got the workflow right, it all went great. But be shure that, if you want to do it this way, you test it a couple of times (also with the editor) before the shoot.
Ps I'm not sure if the 5D has an video sync input. If so, you can sync the video from the clockit too, then you'll really have no sync problems anymore