There's a patch here that appears to allow the user control over the
transmission power of wireless cards via wicontrol:
http://lists.freebsd.org/pipermail/freebsd-current/2003-September/011373.html

It seems fairly old (about a year or so) but it looks useful for situations
where a combination of card and antenna end up going a bit over the legal
limit :/

Has anyone used this yet? And would it be possible for someone with FreeBSD
installed to apply the patch and send me the patched version so I could try
it out, as I don't have FreeBSD installed yet?

Only one problem - apparently the transmission power resets to the default
whenever the driver is restarted. That would need fixing, but it'd be cool
if I could get that working.