In some secret plan we determined a location for subdataset(s) with container(s). In principle such location is arbitrary, but I nevertheless want to propose

.datalad/singularity/<nameofcontainer>

A use case for such a default location would be a convenience option to datalad run that executes a particular command in a specified container (and maybe even a particular version of a given type of container).

What do you think @bpoldrack @yarikoptic ?