Well, until something better is found...
I have a solution that is working for me. I created a batch file that updates the "KFMLastDismissTimestamp" registry entry with the current time, then created a scheduled task to run that script each day. That fools KFM into thinking the nag banner has been dismissed within the last 24 hours, and it does not display it again.
I had first tried setting that timestamp way into the future (like 2025), but that doesn't work -- the nag screen comes back immediately. But setting it daily to the current time seems to do the trick.
The batch file content is attached below if anyone else wants to go this route.
@echo off
setlocal
call :GetEpochTime EPOCH_TIME
reg add HKCU\Software\Microsoft\Office\16.0\Common\KFM /v KFMLastDismissTimestamp /t REG_DWORD /d %EPOCH_TIME% /f
goto :EOF
:GetEpochTime
setlocal enableextensions
for /f %%x in ('wmic path win32_utctime get /format:list ^| findstr "="') do (
set %%x)
set /a z=(14-100%Month%%%100)/12, y=10000%Year%%%10000-z
set /a ut=y*365+y/4-y/100+y/400+(153*(100%Month%%%100+12*z-3)+2)/5+Day-719469
set /a ut=ut*86400+100%Hour%%%100*3600+100%Minute%%%100*60+100%Second%%%100
endlocal & set "%1=%ut%" & goto :EOF
Is the batch file approach still working? It sounds like the best option. For some reason Windows wouldn't run a BAT file but it seemed to accept CMD.
FWIW, in a separate thread I and many others have complained about Microsoft's "backup means we'll take your files out of your control" seize-and-screw strategy with OneDrive. There aren't any MS employees in the forums, just volunteers, so our only "hope" is complaining through the built-in feedback system (though everyone needs to do that anyway!). But even some MVPs have said they don't like what MS did.