Make the butler command in daf_butler able to call the register command in obs_base (at python/lsst/obs/base/instrument.py).
To do this, when obs_base is setup, it should add a colon separated list of CLI python code modules or functions. For example set it to lsst.obs.base.cli.
Then in butler command itself do something like:
and maybe have the imported class import the subcommands to click somehow
...it may help to be explicit and have the env var contain the actual command function (so lsst.obs.base.cli.register or somesuch).