don't know if it's just me, but THERE IS a hook in 'editstyle'...
>> admin_style_form
so your plugin "Prepare Usergroup Selector" can be rebuilt a little, i think... i've recreated the process myself and it's working...
HTML Code:
<plugin active="1">
<title>Restrict Styles to Usergroup: Prepare Usergroup Selector</title>
<hookname>admin_style_form</hookname>
<phpcode><![CDATA[
print_membergroup_row($vbphrase['restrict_to_usergroup'], 'allowedusergroups', 0, array('membergroupids' => $style['allowedusergroups']));
]]></phpcode>
</plugin>