After installing FreeBSD 9.1.0 (i386) on the box, I was able to get the fans under control through this process:
Installing the compat4x-i386 package.
Extracting the contents of http://people.freebsd.org/~jcagle/hpasm-7.22.tar.gz and modifying the INSTALL script to accept my version (ie. changing the if statement to accept '9' as valid rather than just 4 or 5 for these older versions of FreeBSD).
Running the modified INSTALL script and (hey presto) fans are much quieter now. The hpasmd daemon loads automatically on each reboot.
When I attempted to replicate this on pfSense 2.0.3-RELEASE, hpasm installs but hpasmd coredumps when executed.
What this tells me is:
I'd probably need to investigate if the COMPAT_FREEBSD4 flag is enabled in the default 2.0.3 kernel (I assume that it is not);
It is possible to get these fans under control under stock FreeBSD and likley to be possible (if the answer to the point above is no) through custom built pfSense kernel; and
If I wanted to do this on pfSense, I probably wouldn't be able to blindy follow the web gui upgrades without checking that these modifications were preserved.
So, problem (probably :-) solved