OK - to answer my own question…
Based on this post [ http://m0n0.ch/wall/list/showmsg.php?id=73/62 ], I edited the /boot/loader.rc file on the ISO and added the following 2 lines to the very top:
set hw.ata.atapi_dma="0"
set hw.ata.ata_dma="0"
Then burned the ISO and booted - pfSense now loaded in PIO mode and everything worked fine. The setting was retained during the install.
Question for the developers - Will the pfSense update process lose these changes or am I now good until I need to do a new full install?
For reference I used a trial of a program called UltraISO to edit the file. You need to extract the file out of the iso, edit it, then paste it back in and save your new ISO. http://www.ezbsystems.com/ultraiso/index.html