OK, some progress to report:

I reconfigured the M8047 back to default settings (except for baud rates).  Set SLU1 to 300 baud and connected to a Silent 700 terminal.  Powered on and...nothing.  Turns out the 1A fuse on the +12V line on the M8047 was blown.  I don't have any proper replacements right now (Littlefuse PICO II is a good replacement), so I ordered some and temporarily replaced the blown fuse with a jumper.  Now I get console ODT output when I power it on (yay! That means the power-on sequencing circuit is working!) but it will not accept input from the keyboard.  This could be a cabling issue, or possibly a blown UA9637 receiver (or ...).  All I know at this point is that the RRI pin on the UART is stuck at logic 1.

More to come...

...and as promised:

It does appear that the UA9637 is fried.  I can trace the TTY output on the oscilloscope to pin 5 (2 IN-), but there is no change on pin 3 (2 OUT).

Guess I need to make *another* Mouser order...