I am grateful to Roger, W7TZ, who provided information on how to add an external PTT line to the FT-857 so an amplifier could be keyed without giving up the CAT capability on the rear panel.
Here is the information received from W7TZ:

The photos below illustrate how a shielded PTT cable was added to the FT-857 that I use for 6m EME DXpeditions.