Custom "do not have permission" message based on forum?

[sub_ubi]

I have several private forums. I've customized the "do not have permission" message using phrases, but the best I can do is a generic message for all private forums. I'd like to customize the message based on the forum the user is trying to access.

I've tried inserting:

<if condition="$forum[forumid] == 15">Custom message for forum 15</if>

Into phrases with no luck, it displays the message regardless of forum.

Is a custom "do not have permission" message based on forum possible using conditionals? If not, what's your estimate of the level of difficulty would be required for a hack?