[@lveale1983 Participant 2 months, 3 weeks ago]

Just an update on this for anyone else that it’s useful for, I got this working using the filter:
bp_email_set_tokens

add_filter( 'bp_email_set_tokens', 'add_full_name_to_friend_request_email', 11, 3 );

I was able to add a function to create new tokens that could then be used in the emails.

What I am now not sure how to do is to change the friend request notification content, that seems trickier…