To do this before the Issuance rules You'll probably have to modify the IdpInitiatedSignOn.aspx and write some custom code.
Guess you already know about Issuance Rules... But in case someone else doesn't mind Issuance coming after the authentication - or you haven't found a solution...
If you have Exchange or another way (I've done this with Powershell) of building Dynamic Groups based on OU http://technet.microsoft.com/en-us/library/bb123722.aspx you can put them in groups based on their OU. Note if you use Exchange you'll have issues if OU 1 or OU 2 is a child of one another...
You can then (In the relying party claims) create an "Issuance Authorization Rule" to Deny Access to a particular Group SID. You just punch in the group you want to deny, and after they try to authenticate (correct password or no) they get denied. (The message is not pretty.)
They'll get the standard looking "You've misconfigured something" ADFS message with the following info:
Access Denied
Server Name
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