dynamically changing vbulletin permissions/bitfields via a plugin

[dartho]

I want to override permissions using a plugin - specifically for viewing attachments. By default, guests can not view attachments. I know I can set this manually, but would like to do so dynamically.

I think this is the bitfield I need to toggle is:

<bitfields product="vbulletin">
	<bitfielddefs>
		<group name="ugp">
			<group name="forumpermissions">
				<bitfield name="cangetattachment"           group="forum_viewing_permissions"   phrase="can_download_attachments"      install="2,5,6,7"      >4096</bitfield>

I've tried creating a plugin on various hooks with the code:

$userdata->set_bitfield('forumpermissions','cangetattachment',true);

I'm yet to get this working.

the plugin code will have other conditionals (primarily the forum which the post/attachment resides in ), but the short story is I ultimately want to allow viewing of attachments by anyone.

any help appreciated !
Many thanks