This one wound up being something like a take on Sanbitter (which is something like a non-alcoholic Campari & soda) with some intriguing added flavor notes. It was a nice variation from other more citric mocktails so far, veering instead into highly bitter territory but with enough sweetness to balance.
1 1/2 oz mixed berry syrup (leftover from Kader Punch - no shame in recycling)
1 1/2 oz chilled black tea (a fairly mild Darjeeling here)
1/2 oz red grapefruit juice
Stir over ice and top with about 1 1/2 oz each ginger beer (Regatta again) and soda water. Stir again, garnish with a grapefruit twist if you must.
It's too bad that I ended on this one, because it'd be a fantastic apertif. Might have to revisit this one tomorrow.