You should use 10074 to change the active output profile for a current file, eg. MC34 /MCC 10074,0
The global default profile (eg. 10080) is not a good choice for quick testing (it changes the default profile for all files, which is meant to be used to react to manual/external hardware changes)
OK, so I tried it and it works great

For those who would like to implement this, here is what I did:
I set my HDR 3D LUT in my default Output profile (ID 0)
I copied that profile into another Output profile, called 3D LUT Off, that has the ID 1.
I created two batch files to switch between the default profile (3D LUT On) and the one with the LUT disabled (3D LUT Off):
Here is the content of the batch file that enables the 3D LUT:
@ECHO Off
"C:/Program Files/J River/Media Center 34/MC34.exe" /MCC 10074,0
And here is the one that disables it:
@ECHO Off
"C:/Program Files/J River/Media Center 34/MC34.exe" /MCC 10074,1
As you can't run a batch file without having the command line window flashing on briefly (even with the @ECHO OFF command) as you can only minimize shortcuts on launch, I also created two .vbs files calling each batch file so that you can enable/disable the LUT without the flashing, which is distracting and prevents seeing small changes in the picture (for example when paused on a specific frame to see the impact of the LUT):
Here is content of the 3DLUT_On.VBS that enables the LUT:
Set WshShell = CreateObject("WScript.Shell" )
WshShell.Run chr(34) & "[YOURPATH]\MCLUT_On.bat" & Chr(34), 0
Set WshShell = Nothing
And, as you can guess, the content of the 3DLUT_Off.VBS that disables the LUT:
Set WshShell = CreateObject("WScript.Shell" )
WshShell.Run chr(34) & "[YOURPATH]\MCLUT_Off.bat" & Chr(34), 0
Set WshShell = Nothing
This works perfectly, no flash on screen, and the HDR (and/or SDR) LUT is correctly enabled or disabled on the fly, according to both the OSD and the picture. I'll just have to patch this into MCC/Roomie Remote when I find the time.
I was running this on my laptop and the LUT is definitely enabled/disabled, but that's a LUT made for the TV, so I'll do more testing now that I can enable/disable the LUT on the fly and I'll report back, but so far, so good.
The advantage of using MCC commands and profiles is that you can have as many LUT slots as you want, so you could test/use different LUTs.
Thanks all for the help in getting this to work, I find it difficult to evaluate LUTs on an already well-calibrated display without being able to at least enable/disable it on the fly.