So this was a lot easier than trying to figure out how to get an XDMCP login propmt on the new flavors of linux login screen:
On the SGI box Toolchest --> System --> Utilities --> Enable Remote Display
Then on the linux host, in a console window (choose one):
Finally run this command:
Xnest -query host_to_connect -geometry 1280x1024 :1
On the SGI box Toolchest --> System --> Utilities --> Enable Remote Display
Then on the linux host, in a console window (choose one):
yum install xnest
zypper install xnest
apt install xnest
Finally run this command:
Xnest -query host_to_connect -geometry 1280x1024 :1