The mast on my 1980 25+ had a pivoting mast step that allowed the mast to be lowered forward. It came with guy wires to go between the outside shrouds inline with the pivot to the end of the boom. With the guys attached, you could tighten up the topping lift and/or main halyard to the end of the boom, remove the inner shrouds, head stay, and back stay and use the main sheet to lower the mast. I never got around to trying it, but it looked straight forward (like most everything else in theory).
My wife and I went up the mast quite a few times, but our main halyard ran free and I had good self tailing primary winches. I'm not sure if I would be comfortable going up or hauling another person up using the stock barient 10s and a siezed sheave. All things considered, I'd opt for taking the mast down. You could even measure for everything you need, put it back up, and take it down when everything arrives.
You've probably already checked, but just to eliminate the simplest problem to solve, have you tried lubricating the sail slugs or simply raising another line without the sail to verify its a seized sheave and not a dirty track or slugs? I only bring it up because I made this mistake when I first started sailing my 25+. I had arranged for a friend to help me lower the mast to do exactly what your thinking, but when we got down to the boat he brought a can of McLube and a spare sail slug. We wrapped the sail slug with a thin piece of fabric, attached it to the halyard, tied another line to to the halyard and ran it up and down the track. I was amazed at the amount of gunk that came out. we sprayed the track with soap and water and repeated the process. After we felt like we couldn't get any cleaner, I attached the halyard to the mainsail, lubricated the slugs with McLube and hoisted it. It raised and lowered effortlessly.