Hello,
I went through all the docs related to the topic above and I can't find the satisfying answer.
During the time the authenticating client is in ISE unknown posture state you need to create the following: 
1) url-redirect-acl - this ACL defines what traffic triggers redirection 
2) url-redirect - this is https posture portal (client provisioning portal)
 
The two above attributes are mandatory. 
The optional attribute is ACL (dACL) which can limit the IPs/subnets the authenticating device has access to. 
How to do the same on Meraki?
 
For 1) - is this achieved by checking CoA + walled garden (you define here ISE IPs + DNS)? 
For 2) - my assumption is that this attribute is by default understood by Meraki 
For 3) - Filter-ID defined under Access Policy on Meraki needs to be returned in addition on ISE? 
And it needs to match what? Group Policy or Access Policy name?
 
Thank you.