Regsub default vars in log-format

I have log-format with %hr and %r vars and I want to do regsub before logging them.
I tried %[var(,regsub(…)] and %[var(req.r),regsub(…)], but it doesn’t work. Is it possible to use default vars or I need to use captures explicitely? In this case, capture.req.header(i) returns dash for empty header instead of empty line in %hr
I was thinking about lua, but couldn’t find the way to access captured headers