Ok, so the issue with Chrome was that Delegation (--auth-negotiate-delegate-whitelist) was not set, but the normal server-whitelist only (--auth-server-whitelist). This allowed IIS/OpenSSO to function but not SiteMinder.
For Windows the following worked:
"C:\Program Files (x86)\Google\Chrome\Application\chrome.exe" --auth-server-whitelist="*.mykerb.com" --auth-negotiate-delegate-whitelist="*.mykerb.com"
For Mac:
open -n -a 'Google Chrome.app' --args --auth-server-whitelist="*.mykerb.com" --auth-negotiate-delegate-whitelist="*.mykerb.com"