Thanks so much for your reply!
I've been contacting AVAYA/Extremenetworks support and they are not sure if selecting Cisco as vendor will work for Meraki. However, they showed me how to add new vendor and new selection of attribute value pairs specific to that vendor under the Vendors listing. (Does Meraki have a Vendor ID?)
https://gtacknowledge.extremenetworks.com/articles/How_To/000037372
The client currently is using AVAYA IDE as RADIUS server and it's up and running and working fine, so he is not looking to replace it with Microsoft Network Policy Server (NPS).
We are going for live deployment in two days; and I can't risk it if the AVAYA IDE RADIUS doesn't accept Cisco vendor for Meraki.