Thanks to the helpful people on the conference call, I got HylaFAX working today.  It turned out I needed to set not only the "fax_enable_t38" variable, but also the "fax_enable_t38_request" variable.  Further input showed that the best solution was to change the context in the spandsp.conf.xml from "default" to "hylafax", and make a separate "hylafax" dialplan that has this:

<include>
  <context name="hylafax">
    <extension name="FAX" continue="true">
        <action application="set" data="fax_enable_t38=true"/>
        <action application="set" data="fax_enable_t38_request=true"/>
        <action application="t38_gateway"/>
    </extension>

...then the normal dialing rules.

But as usual, I still have problems.  :-(

Now I can't get the old analog FAX machine to successfully send FAXes.  I have it plugged into a cheap ATA, the OBi.  It seemed to be working at one time, but that may have been a fluke.  On calls through it, I have fax_enable_t38 and fax_enable_t38_request set, and I also set "execute_on_answer=t38_gateway peer" before the call is bridged to the gateway.

Should I be using something else?

-- 
Steve