Redfish aggregation across BMCs

Yes, you can absolutely aggregate "satellite BMC" information into a single service - and this is one of the primary design tenets - that you could manage multiple systems from a single service (IP address). The "how" you implement that behind the scenes was not part of the original specification - so yes, a "custom" aggregation mechanism. The desire to perform the configuration tasks for an "external" aggregator led to the AggregationService and the related schemas.

But in the case you're describing, the connections and relationships between the main BMC and the satellites are probably known and well-defined, so that process can all happen without exposing all those details to the public-facing API. If the satellite BMCs are given the "Id" values for their ComputerSystem instances, for example, then their resources can simply be added to the resource tree without name collisions. Even better, you can define a naming pattern up front so that collisions can be avoided without coordination. For example, if this is a blade chassis with "bays" for each server, the "Id" value of the Chassis resource could be "Bay<n>" (1..n) so that they all fit into a aggregated ChassisCollection without modification by the main BMC.

Hope that can get you started, but you're on the right track,

Jeff