Off the top of my head:
You could go into the postbit template (or _legacy depending on which you use) and where it says signature, you could put something like
HTML Code:
<if condition="$post[userid] == X">
<IFRAME border=0 src="http://gd.mlb.com/components/teams/standings/AL_C.html?club=cws" frameBorder=0 width=207 scrolling=no height=188></IFRAME>
<else />
($post[signature] or whatever is there)
</if>