You could customize the automatic messages that are sent out when a member is granted a new trust level on the forum (and the related trust level badge).
The text variables are:
- Trust Level 1 message
system_messages.welcome_tl1_user.text_body_template
system_messages.welcome_tl1_user.subject_template - Trust Level 2 message
system_messages.tl2_promotion_message.text_body_template
system_messages.tl2_promotion_message.subject_template - Trust Level 3 message
lounge_welcome.body
lounge_welcome.title
(I shared a more comprehensive how-to on customizing system messages at Welcome new users with customized trust level promotion messages)
You could also combine that with tweaking the access settings for trust level 3 (Regular) and customizing the default Lounge category.