It's perfectly plausible to custom code independent timescales for objects - even jumping - you just have to adjust the gravity accordingly as well. Gravity is a uniform acceleration applied to all objects, and acceleration is dependent on the timescale. If you're read the article on timedelta, you'll know that adjusting a speed by an acceleration involves speed + acceleration * timedelta
The main reason timescale is global is that it was easy to code
Per-object timescales is possible, but would have to wait until Construct 2.