It appears that libpurple doesn't wait for a <features> tag at all, it just immediately tries to log in. However, when connecting to the google talk xmpp server it instead immediately does a starttls. I haven't looked at their source, but I note that google is sending the <stream> tag and the <features> tag in the very first packet, so perhaps it sees the packet boundary and guesses that my server isn't going to send any features tag at all.
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