loseWriteConnection is the function I had been looking for all day. In retrospect, it was obvious---just look at the ITCPTransport manual page. But, at first I didn't know what I was looking for---I was just confused as to why netcat wasn't working as expected.
I was trying to get server status information which required sending a simple command to the server. When I used a custom netcat-like utility, it worked, but when I used netcat or python/twisted, it didn't. At first, I thought the special utility might have been sending an extra EOF-like character, but some testing eliminated that possibility. Then, I thought it might be a feed-line issue. Nope. Finally, I realized the problem---netcat and python/twisted weren't half-closing the write connection after sending the command. How did I come to this conclusion? I tried the netcat -q option and immediately got back the server status information (before the specified timeout).
Earlier, I had tried to (half-)close the connection with python/twisted using ITransport.loseConnection. But, after fully realizing the half-close issue and making additional loseConnection attempts, I concluded that loseConnection fully closes the connection, losing the response. Next, I found _closeWriteConnection which sounded like it would do exactly what I wanted. The source even looked like it would work, but for whatever reason it didn't. Finally, I was clued-into loseWriteConnection which closed the write-side of the connection while still allowing reading of the server response.
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