There is at least one application I know, `kyua`, which by design knows about this limit and needs to use exactly the same value as the system does -- not to get "permission denied" if a wrong one is used, e.g. if it's set to equal or greater than `prison0` one. The respective kyua jail support patch is work in progress -- https://reviews.freebsd.org/D42350.
If the constant is exposed we could avoid its definition duplication within the src tree.
Do we have objections against?