Well this bug is 6 year old, so I don't know if anyone still cares,
anyway I have a simple proposal:
Use xmessage to display the required information.
The attached patch modify the scripts used by xdm in order to do so.
However I don't think it can be integrated as such, mainly because it
uses xmessage, killall and lsb_release without checking if the
corresponding command exist. Also one might want to tweek the look of
the xmessage window using some xresource file.
More information on the used techniques are available from the gentoo
wiki [1].
1: http://www.gentoo-wiki.info/XDM/Customization