Dthrow -> Uair does work after about 40%. If you buffer jump, it won't work. You have to jump right after the animation finishes (I keep Roy in my peripheral vision and jump after the ground shockwave goes away).
At about 40-55% (?), you only need to SH to connect, although you might sourspot it. At 55%+, you have to double jump, but if you buffer the double jump then Uair it should connect, again maybe sourspotting.
From my experience this combo works up until around 160%, which is kill range for Uair. Also, Roy doesn't have any Dthrow true combos at 0% in any weight class, as far as I can tell.
I've tested some more and it seems that Dthrow -> sourspotted Uair does work at around 100%. I have definitely done true combos lower than that, but I can't get it to work to confirm. You can't buffer the jump, like I said before. Wait for the first moment you can move (not sure if it's an IASA or just the end of the animation & lag) and jump, double jump if you have to. Immediately after your last jump, Uair and you'll get it. Of course, do all this as fast as possible.
One of the first combos I discovered with Roy was first hit nair into Jab. Glad to see some more with nair!