Ok, I had a look today.
The L VOR receiver does utilise it's built in Marker module and sends the audio signal through to all the audio control panels. The work is done inside the ACP.
On the ones with no seperate marker control the audio is fed to the L/C/R ILS/MLS positions and that is how you control the marker audio.
The manual revision for aircraft with these panels fitted states
"The AVTECH Audio Select Panels on ** 757 do not have a Marker control switch. Marker Audio is obtained by selection ILS control switch. "
Incidentally the 767 aircraft I looked at today had the same control panels - ie no seperate marker control.
Hope this clears things up
