More generally, I've seen some ancient SunTour rear sprockets that are 38 teeth. It would take some surgical procedures to put one on an existing cassette or freewheel, but it gets you past 34. What rear derailleur/dropout combination can accommodate that, I don't know. You might find yourself getting into a gearing/derailleur combination that is unique, ancient, and very difficult to find a full group in good condition, and that might not even deliver what you expect on some frames.
If you need more range past the currently available 11/32 10-speed system which can be purchased off the shelf, I think you need to address the chainrings as well as the rear end. If you need to stick with the 7-speed spin-on freewheel, you can find a 13-34 Megarange, or if you hunt a 13-32 Sachs-Maillard 7-speed freewheel. SunTour also made some similar wide-range combinations. The Sachs and Suntour units usually had an even gear spread across the range, not the roadie-spread-plus-deep-bailout that the Megarange represents.