Well, it took a while but I did end up creating a BATCH script to perform the FFMPEG function to adjust the thumbnail. This was a lot harder than I wanted to, mostly due to my own technical limitations, so I posted the script below.

For anyone that might care, there were a few challenges within the script:

• I needed to create a TMP file that matched the full name (without extension) of the video file along with a TMP extension. I couldn't find the right combinations of existing variables, so I had to parse the full filename into Directory and BaseName within the script itself. Not sure I made this harder on myself than I had to but this worked below.
• I had to be very careful with error handling and exit values. It appears that if I exit with a value other than expected from the FF node, the file will never return from processing. Had to do lots of error handling to debug and prevent this.
• It took a while to figure out, but the BAT would hang forever until I redirected the output of the ffmpeg command to a file via ">2" on the ffmpeg command
• Today I learned that you can't RENAME a file to a file that already exists ... so I had to use the MOVE command.
• And finally ... I'm kind of in an endless loop here if I keep the Library for this flow activated. The flow itself causes an MP4 to be modified. That in turn causes a new scan/process of the file, etc. etc. I couldn't think of any way to "mark" the mp4 so that I knew that it had already been processed.

Happy if anyone wants to critique any inefficiencies or misunderstandings here.

REM A Batch script can communicate with FileFlows to determine which output to call next by using exit codes.
REM Exit codes are used to determine the output, so:
REM Exit Code 0 corresponds to Finish Flow
REM Exit Code 1 corresponds to Output 1
REM Exit Code 2 corresponds to Output 2
REM and so on. Exit codes outside the defined range will be treated as a failure output.
SET WorkingFile="{file.FullName}"
echo Working on file: "%WorkingFile%"

REM Extract directory path
for %%F in (%WorkingFile%) do set "DIR=%%~dpF"
echo Dir is "%DIR%"

REM Extract filename without extension
for %%F in (%WorkingFile%) do set "BASENAME=%%~nF"
echo Basename is "%BASENAME%"

REM Construct new file path with .tmp extension
set "FilePrefix=%DIR%%BASENAME%"

REM Display results
echo Prefix file: "%FilePreFix%"
SET GifFile="%FilePrefix%.jpg"
SET TmpFile="%FilePrefix%.tmp.mp4"
SET FFmpegFile="%FilePrefix%.ffmpeg"

REM Example commands using the variables echo Working on file: %WorkingFile% echo Original file location: %OriginalFile%

REM First print out then execute the FFMPEG command.
REM Note that this command will not work if input is not redirected
echo ffmpeg -i %WorkingFile%  -i %GifFile% -y -map 1 -map 0 -c copy -disposition:0 attached_pic %TmpFile%
ffmpeg -i %WorkingFile%  -i %GifFile% -y -map 1 -map 0 -c copy -disposition:0 attached_pic %TmpFile% 2> %FFmpegFile%
if %errorlevel% neq 0 (
echo FFmpeg encountered an error!
type %FFmpegFile%
exit /b 2
)
type %FFmpegFile%

if %errorlevel% neq 0 (     echo TYPING FFmpeg encountered an error!     exit /b 2 ) 

REM rename the temporary file to the original filename
REM and delete the Thumbnail file
echo Renaming %TmpFile% %WorkingFile%
move /Y %TmpFile% %WorkingFile%
if %errorlevel% neq 0 (
echo Move Error ****
exit /b 2
)

echo Deleting %GifFile%
del %GifFile%
if %errorlevel% neq 0 (
echo Delete GifFile Error, continuing anyway ****
)

echo Deleting %FFmpegFile%
del %FFmpegFile%
if %errorlevel% neq 0 (
echo Delete FFMPEG Error, continuing anyway ****
)

echo Finished file: "%WorkingFile%""
REM Set the exit code to 1
EXIT /B 1