mtlog--thanks for the response.
If the appliance (pan under the airhandler) is trapped, dumps into a tailpiece and then goes through the shower/tub trap, I believe the appliance would be double trapped but not the plumbing fixture. However, M1411.3.1 on condensate drains addresses pipe size and grade but I can't find any requirement that it be trapped.
The biggest question in my mind was whether or not any of the plumbing codes prohibiting tying the condensate drain into the tail piece of a shower or tub fixture. To me it is the best design as it is a proper disposal of the water, wouldn't depend on a pump (unless it had to be lifted) and eliminates the possibility of sewer gas (unless you never use the tub/shower).