BEP 011 (Structural Derivatives) introduces a volspace entity that could be useful here.

EDIT: Unfortunately, at least one entity (cohort) could end up overloaded. For example, if I wanted to use dhcpAsym:cohort-42 as the surface space and MNIInfant:cohort-1 as the volume space, cohort ends up having two values (42 and 1) and no way to associate the values with the right templates. I think we'll need a volcohort entity to make it work.

I know it is not BIDS compliant, but this seems like a good argument for supporting the + character as a cohort extension to greatly simplify naming.

dhcpAsym+42, MNIInfant+1

That's a great point. I've been working on using volspace and volcohort in a branch, but the + character might be better.

It seems like folks would prefer to use +s in the space entity instead of adding new cohort, volspace, and volcohort entities. So something like "dhcpAsym:cohort-42 as the surface space and MNIInfant:cohort-1 as the volume space" would end up as space-dhcpAsym42+MNIInfant1.

Niworkflows is still going to need to parse space definitions that include the surface and volume spaces, so I think I can still move forward with #883.