[powercf]

That is probably a very rare case, and any shell-agnostic-completion-protocol could be extensible to handle such a case. But, I think 99% of completions that people want/use on a daily basis could be covered by a machine-readable version of the usual "--help" option (with support for placement of paths, pids, strings, numbers etc.).

[loa-in-backup]

If it doesn't work reliably for all cases, there would soon be inevitable need for the kind of scripted completion we have now.