This is already possible with a --config option to either prosody (since 0.6.0) or prosodyctl (since 0.8.0 I think).
Does this solve your problem?
@Zash, yes, it does solve *my* problem, indeed, thank you. :-)
However, maybe the subject of this issue should be adding this to information to:
3) prosodyctl --help,
Otherwise, how can users know about it, other than reading source code?