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The way I saw it done is with Process to Process messaging. The process to recall sends out a message to the PI that is holding the task for B, and that process is killed. No more task, and A can proceed to make changes, then create a brand new submit to B.
Process to Process is very inefficient and slow, but this is still, I think, a very good use case for it. It's very rarely used, and performance isn't critical for this type of operation.
There may be a better way of doing it, but I haven't been tasked with implementing it, thankfully.