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What I would like to do (and is running on the Spoke for the Spoke-to-Hub direction) is have one tunnel have a dedicated 1.25 Mbps, while the others share the remaining 3.75 Mbps, each with a guaranteed minimum of 1.25 Mbps - is this possible?
As far as I know, Cisco doesn't have a feature to share a known aggregate bandwidth between spokes. (I.e. spokes sharing their current bandwidth utilization to the hub with the other spokes so their current aggregate doesn't exceed the hub's bandwidth.)
If the router on the other side of the hub link supported QoS, then you wouldn't need to shape the spokes. (NB: often available with MPLS clouds.)
Otherwise, there's a recent new feature in DMVPN, where tunnels will dynamically shape based on concurrent IP SLA tests. If IP SLA sees a slowdown, the router assumes there's congestion, and shapes its egress slower. In theory, this should provide results to what you're hoping to do. In practice, I haven't tried this feature and don't know how "laggy" it is. Might be too slow to react if trying to support something like VoIP.
