This bounty has been claimed by Daniel S. Haischt, who sent us some patched files, from which we were able to create a modification with a more minimal impact.
To add this feature, mount the filesystem in a writeable mode (if not already) with:
mount -w /
and then add the following code to /etc/inc/services.inc, starting at line 238:
if ($dhcpifconf['tftp-server-name'])
$dhcpdconf .= " option tftp-server-name \"{#dhcpifconf['tftp-server-name']}\";\n";
Then just change your config.xml and add
<tftp-server-name>X.X.X.X</tftp-server-name>
under pfsense/dhcpd/lan. That should do it.
I agree with robbyt that adding the ability to set DHCP options is a common thing that is not supporting a 5% minority but actually would be used by a lot of people, and the changes to the interface would be minimal.
Best wishes,
Carl