Not a weight restriction, but a size restriction.
Forbidden Journey and Dragon Challenge are rather limiting as to what size of body can go on them. Forbidden Journey modified the two outside seats (on a bench of 4 seats) on each ride vehicle so that the restraint allows a bigger person on it. But that still has limits.
There are test seats outside both of those rides, and many people use them. If you are uncomfortable, just play it off as though you're taking silly pictures. It's better to find out outside rather than inside, when the walkway is moving and you have to work it out and maybe have to get off the ride quickly. Then you can come to terms with it, if you don't get to ride, and enjoy the walkthrough of the line. All Universal lines can be walked through by anyone, so you can enjoy your time inside the castle or inside the grounds of wherever Dragon Challenge is supposed to be, and then exit right near the end instead of riding.
To give an example, my husband is 5'10" and built like a sumo. He has muscle everywhere and has weight distributed everywhere. He's about 340 still (trying so hard to lose, but has a pituitary tumor that makes it hard) He has a particularly broad and deep chest. He can barely even scoot back in the FJ ride seat, and definitely can't close the restraint, even in the modified seats. Dragon Challenge remains an impossible dream for now.
If he were taller his weight would be stretched out, and it might be possible. If he didn't have big legs, it might be possible to bring the restraint on FJ down a bit more; but the way he is built means the tops of his legs get in the way along with his chest and belly.
I hope that helps. From watching DH deal with this, and he's a very very emotional guy (see the pituitary tumor mention above...it creates a hormone that increases emotions), it seems that it's best to take the feelings out of it, see if you can fit in the test seats, and try to make the best of the day if you can't. DH still loves Universal even though he can't yet ride any of the big rides, so it's still possible to enjoy yourself even without those rides.
And no one knows how the attractions in Diagon Alley are going to be.
Nor do we know exactly when they will open.