Can you lock it down with IP security (IIS7 +)?
<location path="elmah.axd">
<system.web>
<httpHandlers>
<add verb="POST,GET,HEAD" path="elmah.axd" type="Elmah.ErrorLogPageFactory, Elmah" />
</httpHandlers>
<!--
See http://code.google.com/p/elmah/wiki/SecuringErrorLogPages for
more information on using ASP.NET authorization securing ELMAH.
-->
</system.web>
<system.webServer>
<security>
<ipSecurity allowUnlisted="false" >
<add ipAddress="127.0.0.1" allowed="true"/>
</ipSecurity>
</security>
<handlers>
<add name="ELMAH" verb="POST,GET,HEAD" path="elmah.axd" type="Elmah.ErrorLogPageFactory, Elmah" preCondition="integratedMode" />
</handlers>
</system.webServer>
</location>