Yes, SECCOMP_RET_TRACE works, but nsjail doesn't have code to support that - it didn't seem that useful when mount namespaces can police access to files.

Otherwise, it's possible to make it support that. Though, a word of caution: ptrace() is complex, and sometimes buggy interface with a lot of corner-cases - iow: it's easy to make a mistake with consequences for security of the whole setup.

PS: It's possible to use SECCOMP_RET_TRAP (TRAP(number) in kafel's - nsjail seccomp-bpf cfg language - nomenclature), and rewrite syscalls in-process with help of SIGSYS signal handler.