Without knowing how ACS3.x code is shared between sub.foo.com and foo.com (or much else for that matter), here's a suggestion to try at your own risk:
rename the sub.foo.com/board/q-and-a-fetch-msg.tcl file to something like: sub.foo.com/board/q-and-a-really-fetch-msg.tcl
create a sub.foo.com/board/q-and-a-fetch-msg.tcl file that returns html something like:
<frameset rows="*" cols="185,*" border="3" framespacing="3">
<frame SRC="partners-left-navbar-example.html">
<frame src="http://sub.foo.com/board/q-and-a-really-fetch-msg.tcl">
</frameset>
<noframes>This page uses frames. <a HREF="partners-left-navbar-example.html">
See the rest of the partner site</a>
or go check out
<a href="sub.foo.com/board/q-and-a-really-fetch-msg.tcl">the message</a>
you were wanting to see.</noframes>
