We do fall back to BITS when agents are not able to download patches, but no throttling is involved. From what I recall, incorporating BITS throttling is a costly (R&D) endeavor and we haven't see enough requests for it in the past. I think your best bet would be to submit a feature request to let the Product Managers know users are looking for this feature.
That being said, I know of a few customers who chose to use 3rd party file replication or copying applications (that have throttling) sync files between the Distribution Servers instead of allowing Protect to do it. This is in extreme cases where bandwidth is very limited.