We had tried this once and due to links falling out of the group and increased latency and response time, pulled all IMA out of our network. We switched to hardware IMUX instead, which was much better.
By connectiong Router B to the 192.168 network you are now creating the situation where devices on any network can communicate with devices on any other network. So if you don't want that to happen, they will be adding access lists to all the router...
I assume you tested this from the other end of the serial link where the access list is? So I ask are you using CEF (Cisco Express Forwarding) on this interface? You must.
