I’d love to be able to type ///a and ///r into the Repeats Popover as well as ///att and ///rel to achieve these notations.
That sounds interesting, may I ask what this notation is meant to look like? Is the goal so that a player fades in or out of tremolo (I’m picturing like attack/release on a synth)?
Wait WHAT?! 
I didn’t know there was an actual term for that – I create that notation all the time manually inserting separate notes and then tying them etc etc. Lucky day to learn something new! And I agree, it would be nice to make it even faster with your recommended syntax 
Tremolo attack/release was added in Dorico 4.3 to make it easier to show tremolos on tie chains that don’t appear on either the first or last stem (because previously, that required a switch to Engrave mode, selecting the first/last note in the tie chain, and using the Single stem tremolo property).