Adding strongback bracing to an existing floor truss
I've built quite a few homes with floor trusses, and we didn't always have the foresight or opportunity to lay in strongbacks during the truss set. Have you tried getting longer 2x6 boards in there? You might be surprised how much they flex. I'd bet you can get 16-footers in there without much issue.
I'd start with that. Just don't let your grip slip and smack yourself in the chin. Once you have one in place, use it up against the top chords as a slider for the next ones.
Otherwise, you're onto something with the OSB. As you probably know, OSB is what makes up the vertical component of TJI joists, meaning it has plenty of strength on that axis to act as a strongback. You could rip 8-10&quot; widths from 1/2&quot;x8' sheets, flex them into place, and stagger the butt joints of two layers by two trusses. I have confidence that this would do at least as well as 2x6 lumber.
Actually, 7/16&quot;x10' OSB is commonly available. If you shortened them to 115.2&quot; you'd span one more joist.
FYI, such strongbacks are usually set against the top chord. By keeping them tight in all cases you ensure a flat floor. (Even engineered trusses can have some crown or wiggle to them.) If you have the option I'd move your ducts down and do that. It might help to have them temporarily out of the way anyway. Otherwise it probably doesn't matter much at this point.