My initial response to this was "not supported on so many levels"; however, after a little reflection I should be fair and flesh it out:
First, let me make sure I understand what you are trying to accomplish:
You want to write a CAD workflow on the Answer event that automatically starts an ad-hoc conference with another DN. That DN is assigned to an ATA which is auto-answering. The FXS port on that ATA has some type of analog recording device attached to it. This works fine as long as the DN is not a shared line with some other device.
Assuming I understand you properly:
1) Why would you have a shared line that is set to auto answer? This seems contradictory to me.
2) Make absolutely sure that this is _not_ the same DN as the agent's IPCC Extension. Shared lines are never supported for IPCC Extensions.
3) Why do you feel that the agent needs control over the DN on the ATA?
4) Why on earth would you need to do this? At a minimum you should be able to use the built-in recording within CAD and write a workflow that automatically starts this.
5) I haven't tried this but I suspect it would make historical reporting less usable as there will be a ton of additional call detail records.