ls-refs protocol extension

Hi list,
I have been working on a custom git server where the sha values
required to respond to a ls-refs command can occasionally be expensive
to compute.
It would be a great improvement of user experience if it was possible
to show progress to the user while this is happening.
I was hoping that this could be solved by using the already existing
sideband-all capability, but unfortunately it seems that even with
sideband-all the sideband is not available during ls-refs.
So my plan would be to start working on a patch to add sideband
support to ls-refs, unless there is
some other protocol feature that I have overlooked and that would
enable my use case.

Any pointers to either alternative solutions and/or hints to potential
pitfalls in the implementation
of my suggestion would be greatly appreciated.

Best regards,
Christian Schilling