There are a few tricks to getting a boozy tea add-in right:
(1) I like to stick to herbal teas when mixing, because the tannins in black tea can skew really wonky and astringent when liquor is present. Chamomile honestly mixes well with everything, so make sure you've got some on the table.
(2) For heaven's sake, don't eyeball your pours unless you want your guests leaving sloshed. An ounce of spirit per cup (or 1.5 oz if you really want to up the toddy vibes) is generally enough, so put a jigger out that fits that measurement.
(3) Lemon, sugar, and honey -- friends of tea -- are also friends to a toddy. Add some to your cup according to preference; I love that each guest can adjust the sweetness to exactly the way she likes it.
(4) Don't limit yourself to the regular characters (whiskey, gin, rum) as add-ins. Liqueurs and amari work well in tea, too -- Benedictine, Yellow Chartreuse, and St. Germain are a few of my favorites.
(5) Finally, you don't want to concentrate evaporating alcohol vapor into your nose (ethanol evaporates at a lower temp than water), or the drink will seem much boozier than it really is. Whenever you serve a warm alcoholic drink, it's important to use a wide mouth teacup (the wider the better, like these from Villeroy and Boch) to disperse the vapors, NOT a steep sided coffee mug.