After some experimenting I found a way to do length tuning of traces by using the PCB tab > <All Nets> filter to see the routed length of nets on the board. From there I can make a change to a track length and re-evaluate the length afterward.
But this is horrifically fragile and involves a ton of rip-up for even the smallest change. High-speed differential signals are common now (USB D+/D-) even in low-end designs. Can interactive length tuning be added to this future roadmap I've been hearing about? I don't see the point of advertising CS supports differential pair routing if length tuning is not possible.