Please add support for HostResourceGroupArn in the Placement object. This would allow Jenkins EC2 plugin to provision an EC2 Mac instance on to a specific Host Resource Group. If this Host Resource Group doesn't have a free EC2 Mac dedicated host it will actually allocate one automatically. It will also release that EC2 Mac host after 24 hours if there isn't any Mac instances running on it.
So by adding support for this parameter would allow EC2 Plugin to dynamically launch EC2 Macs without the need to have a pre provisioned fleet of unused EC2 Macs.
Host Resource Groups is a feature of AWS License Manager: https://docs.aws.amazon.com/license-manager/latest/userguide/host-resource-groups.html
You can use it without the need of having specific licenses to manage a group of dedicated hosts in the case of EC2 Macs. It supports automatic allocation and release of EC2 Macs.