Header and footer are filtered. You likely will not be allowed to use comments like that. You can try the below to see if it'll allow your comments to function.
That worked - thanks!
Reg. #3 = your answer: I'm not sure I follow this here - let me pls rephrase it so that you can confirm if I understand it correctly: are you saying that in invites plugin, under parameters > email: in body header field I just enter the MY_INVITE_HEADER - like pictured on attachment? that would make sense since all other configuration fields (such as registration) are treated that way... it didn't occur to me initially that I can do the same in plugins as in config
that's pretty good uncorking! Thanks!