Enforcing asymmetric TCP MSS?

Hi,

I'm trying to use scrub max-mss rules to create asymmetric MSS's.

Is this supported?  So far, I haven't got it to work (hence my post here).
The machine is running OpenBSD 4.9 with 2 network cards.

I have been trying things like:
match out on $ext proto tcp scrub(max-mss 1000) flags S/SA
match in on $ext proto tcp scrub(max-mss 500) flags SA/SA

Scrubbing on the other interface doesn't seem to work either.
match out on $int proto tcp scrub(max-mss 500) flags SA/SA

client <--> pf gateway <--> internet

With a sniffer on the client link and the internet link,
-client sends large MSS on initial syn (> 1000)
-pf scrubs MSS to 1000 on initial syn

-pf receives large MSS on syn+ack (> 1000)
-pf scrubs MSS to 1000 in syn+ack reply to client

Thanks!