I’m looking at buying an old 26er kona unit frame for my new single speed commuter and am just wondering if the sliding drop outs would give enough clearance to fit a 650b wheel in and still keep the chain taught. Has anyone tried this?

If it has an ‘A’-type seat-stay setup, which a Kona does have, then there should be plenty of room below the bridge. If you have access to a 29er or 700C wheel, try putting that in to see the clearance, a 650B wheel should be roughly between that and a 26er. I’d love to put 650B wheels on my two hardtails, but they both have wishbone stays, and there isn’t much space above a 26er, let alone a 650B.