Smoke test an AC system? That only works if there is a gross leak, not a minor leak like you describe. AC systems leaks are so small, because of the high presures involved, that a dye is the only good test. And lots of dye!

You could be leaking from a number of areas and probably on the high pressure side of the circuit. An evaporator leak is the hardest to find only because it cannot be inspected, I would look first at the most obvious places in the engine bay:

Coolant hose going from the compressor to the condenser along the bottom of the car.
Accumulator/dryer bottle can crack or get porous.
If you leave the AC switched off for long periods of time, the compresor seals can dry out and start leaking.

Find a new AC shop. One that deals mainly in auto AC, not an engine repair facility. Loosing 6 ounces a year is not much, most of us loose 6 ounces a month.

Klaus
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I still miss my 164 and my 854T. Just driving a V70R :)