I don't think Tasker can control the default there, but it can help some. Using Tasker with Bluetooth AutoConnect, I can enable/disable the phone handling part of the Bluetooth connection. If I am at home, or connected to my car's built in Bluetooth, I don't want to talk to my watch, so I disable then. When I'm out walking around, I'm more likely to want to answer with the watch, so I enable then. When its disabled, I use a custom Tasker scene to do caller id as well as answer and hangup buttons on the watch.
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