Are you thinking narrowly about "What changes to the Plan 9 kernel would you make to emulate the Linux openat() system call" or more generally about "How would you design a facility for plan 9 that provides an equivalent service?
As I understand it from the rationale section on the linux man page, the call exists to avoid a race condition between checking that a directory exists and doing something to a path containing it. An additional motivator is providing the effect of additional current working directories notably for Linux threads (which presumably don't have their own. I think 'threads' (processes that share memory) on Plan 9 do???).
This is all based on the assumption that holding a file/directory open keeps it alive and in existence... which on Plan 9, I think it doesn't, does it? As I understand it, remove can remove a file or directory that is open, which is not like UNIX/Linux...
Sorry, I'm just trying to understand the question.