xdm pre-hook?

[Kurt Granroth]

I have a laptop with an ATI chipset.  I use this laptop with the
internal monitor (1400x1050), a Dell 19" (1280x1024), and an HP 20"
(1600x1200).  There is no one xorg configuration that an take care of
this combination so I was forced to create three separate ServerLayout
sections with the appropriate monitor settings.  X absolutely will not
"re-detect" a monitor while running, so there is no way for me to switch
monitors without restarting X.  I'm not thrilled about that, but I've
made my peace.

Okay.. that's the back-story.  I am now trying to figure out a way to
dynamically use different monitor settings after an X restart.  For
awhile, I was forgoing xdm and starting from a console.  That way, I
could do something like 'startx -- -layout Dell' or 'startx -- -layout
HP' and I would get my customized settings.  I switched to using xdm
(kdm, actually) recently and have been so-far unable to figure out how
to do something similar.

I keep thinking that an xdm "pre-hook" command would be perfect. 
Basically, if xdm called some command or script just before starting up
X, then I could hi-jack that and have it switch out my xorg.conf to
whatever monitor I'm using.  Does anybody here know if there is such a
thing?

Another thought involves tracking down how xdm starts up X itself.  If
it's hidden in some script, maybe I could hi-jack that and substitute my
own -layout params.  Or is X hard-coded inside of xdm?

Anybody have any ideas on how to do this?

Kurt