>> Would a reasonable way to do this be to add a toggleable option to the defcustom for display-buffer-alist, and
>> document this in async-shell-command's docstring?
>
> I think it could be.  Martin, WDYT?

What is a toggleable option?  A new action function?  Or simply adding
‘display-buffer-no-window’ to the value menu?  Or something completely
different?