here are some of the config.xml snippets that i used to make this work (this is not my original work – originally done by Chris Buechler at/for m0n0wall):

<interfaces>
               <lan>
                        <if>fxp0</if>
                        <ipaddr>192.168.0.250</ipaddr>
                        <subnet>24</subnet>
                        <media/>
                        <mediaopt/>
                        <bandwidth>100</bandwidth>
                        <bandwidthtype>Mb</bandwidthtype>
                        <bridge/>
                </lan>
                <wan>
                        <if>fxp1</if>
                        <mtu/>
                        <gateway/>
                        <blockpriv/>
                        <media/>
                        <mediaopt/>
                        <bandwidth>10</bandwidth>
                        <bandwidthtype>Mb</bandwidthtype>
                        <spoofmac/>
                        <schedulertype>priq</schedulertype>
                        <ipaddr>dhcp</ipaddr>
                        <dhcphostname/>
                </wan>
                <opt1>
                        <if>fxp2</if>
                        <descr>OPT1</descr>
                        <bridge/>
                        <enable/>
                        <bandwidth>15</bandwidth>
                        <bandwidthtype>Kb</bandwidthtype>
                        <ipaddr>192.168.0.254</ipaddr>
                        <subnet>24</subnet>
                        <gateway/>
                        <spoofmac>01:02:03:04:05:ff</spoofmac>
                        <mtu>1500</mtu>
                </opt1>

once those spoofmac lines are added in – just reboot and assign your devices like normal. once done it will spit out the mac addy’s (actual).