I would like to apply a filter for resources from a particular subscription.
I.e. only get service users which belong to a given POA subscription, or webspaces from that subscription only, etc.
This is not possible. POA starting with 5.5.4 has account wide service users, it does not make sense to filter them by subscription anymore. Services from multiple subscriptions can be consumed by any user.
If you need to associate your resource with a certain subscription, you can add a relation to the POA subscription resource in it.