automount subdir of mounted fs

[Antoine Jacoutot <ajacoutot_(_at_)_lphp_(_dot_)_org>]

Hi :)

One quick question.
I know that what I want to do is possible using am-utils but I cannot 
get it working with the 4.4 BSD automounter shipped with OpenBSD-3.5.
Basically, before trying again for hours, I wanted to know if it was 
possible at all to automount a directory which is a subdir of a mounted 
filesystem.
For example, I would like to automount /usr/ports (/usr is a regular 
local ufs mounted filesystem) with amd... so far as soon as I start amd, 
the content of my /usr directory disappear ; it looks like amd needs the 
whole mount point.
It works with no problem for mount points like /home, /net...
I read http://www.netbsd.org/Documentation/bsd/amdref.html as well as 
the included documentation without finding anything about it. The closer 
I got was with using type=link, but this does not seem to work either.

Any hints on that issue would be more than welcome.
Regards,

Antoine