I abandoned my effort at injecting a short 2600 Hz "wink" because it was
too loud and hurt the ear. Perhaps an amplitude parameter might be in
order for tone_stream?

Ask and ye shall receive! You just needed to RTFM in the right place:

http://wiki.freeswitch.org/wiki/TGML

Note the "v" param. An example of using it is down in the Perl example of the bong tone.

-MC