I will add CAP support for this option to the next version. However, ISUPPORT changes the way a client works in many different ways. A bouncer should ideally wait for a few seconds after the MOTD is sent (which is what mIRC does) before initiating methods such as joining channels. This allows a client to process both CAP and ISUPPORT properly. The only other option would be to move every ISUPPORT feature into CAP to prevent similar issues in future.
