Revisiting this post.
The previous posters question is valid. A clean design would have AP-Groups exist only in the Cluster they are meant for. This however would not allow ADP to work properly if Aruba-Master resolves to a cluster that does not contain the AP-Group with LMS pointing to a different/correct Cluster.
I see two options:
1- Create the AP-Groups at a higher level as previously suggested. All Clusters would then have all AP-Groups (not clean) but this would allow them to learn the correct LMS.
2- Use DHCP Options 60 and 43 which has higher priority than Aruba-Master.
If there are better options, please post!