Ok Jim, here's an unexpected result...
I built the SMD version of the 6-transistor design (found on the ADMS page here) and everything checked out fine, even with the Loopback test app I found on your site. I set S1 to COM1 and it's not even detecting the interface when I go to connect (gives the S1 not connected error after about 1.5 seconds). I tested this interface with and without the 4k resistor and the concealed display w/ line duplexer and still nothing.
Has the 6-transistor design been tested with S1 before? I was under the impression that the ADMS design was pretty much the exact same as the CT29A cable.
I'm thinking that maybe I need to send one of you 1.8 V1 owners one of my interfaces to see if maybe my V1 just isn't working right.