I believe this work is valuable. *Possibly unactionable discussion*: The most obvious use case, to me, is a mobile client trying to conserve power by limiting the number of acks it sends on a download. It therefore seemed counter-intuitive to have this be a data-sender-driven process. However, as the real limiting factor is the flexibility of the sender's congestion control and loss recovery, I eventually saw that your chosen design is more or less inevitable.
I wonder how to implement a server when this extension is ubiquitous. Is the correct strategy to send ACK_FREQUENCY with quite large thresholds and Requested max_ack_delay (and, presumably, also a quite high value of L for Appropriate Byte Counting), so that a client sensitive to these can go as high as it needs to? *Nits*: Please specify the acceptable packet types for each frame. Please define what it means to "bundle" an acknowledgment.
