I was looking for something similar myself and found solution:
Download Template Conditionals
http://mybbhacks.zingaburga.com/showthread.php?tid=464 if you haven't, activate it, then use a code similar to this (replace whole template code):
<tr>
<td class="trow1" style="vertical-align: top"><strong>{$lang->post_icon}</strong></td>
<if in_array($GLOBALS['fid'], array(4,6,7)) then>
<td class="trow1" valign="top">{$iconlist}<input type="radio" class="radio" name="icon" value="<if $GLOBALS['fid'] == 4 then>1<elseif $GLOBALS['fid'] == 6 then>2<elseif $GLOBALS['fid'] == 7 then>3<else>-1</if>" checked="checked" style="display: none;" /></td>
<else>
<td class="trow1" valign="top">{$iconlist}<span class="smalltext"><label><input type="radio" class="radio" name="icon" value="-1"{$no_icons_checked} />{$lang->no_post_icon}</label></span></td>
</if>
</tr>
In this code forums with ids 4,6,7 have default post icons, the rest has visible "no icon" selected. Forum with id 4 has default icon with iid 1, forum with id 6 - iid 2, forum with id 7 - iid 3.
So to add more/other forums change array(4,6,7) to your own comma separated fid list, then edit the <if $GLOBALS['fid'] == 4 then>1<elseif $GLOBALS['fid'] == 6 then>2<elseif $GLOBALS['fid'] == 7 then>3<else>-1</if> adequately.
@down, the same template as in tutorial, so posticons.