VT+KVM: is there a way to disable video output on a given connector ?

Hello,

I have an old Dell Inspirao 9300 with a Radeon video card, which I use
to do some tests.

This computer has a screen (1920x1200) in very bad state, so I use it
with an external display (1920x1080), switching to the external by a
special key combination (Fn+F8).

This works OK when the video is in alphanumeric mode, that is with
syscons. But with the video in graphics mode, both screens are used.

Under X, the output to the internal screen may be suppressed with
xrandr. But with vt I have not found any documented tunable/sysctl to
achieve the same effect (I already use the kern.vt.fb.default_mode
tunable to force the resolution).

Thanks for your attention,

CBu