This may not work for you; but it works for me using Safari on an iPad:
I have saved the 'Look & Layout page' of my profile as a favourite. When I get to a thread that I want to change the order of, I
1. Open a new browser tab (which automatically shows the favourites)
2. Select the "Profile" page saved previously and select the order I want the thread displayed as
3. Select the previous tab, showing the thread I'm interested in
4. Hit the "reload" thingy, and voila!
When I've done with that thread I select the "Profile" tab and revert to previous settings.
It might be rather inelegant, but it works for me!