It's possible. Take a look at the
bb_plugins.php code of
parseUserInfo_user_custom1 function which says:
if($av!='' and $av!='*') $im="{$a1}<img src=\"{$GLOBALS['main_url']
}/img/forum_avatars/{$av}\" alt=\"{$alt}\" title=\"{$alt}\"{$avatarDim} />{$a2}";
elseif($av!='' and $av=='*') $im="{$a1}<img src=\"{$GLOBALS['avatarUrl']}/{$GLOBALS['cols'][0]}.mbb\" alt=\"{$alt}\" title=\"{$alt}\"{$avatarDim} />{$a2}";
else $im='';
The first string of code handles pre-loaded avatars;
the second string handles uploaded avatars;
the third string handles nothing if no pre-loaded or uploaded avatar is used.
For example, instead of empty string ('') you may put an HTML pointing to your default image.
Let us know if it helped.