It seems the mullet idea (according to the last link) doesn't make such a big jump (26-29). More like 26-27.5 or 27.5-29. I get that a bit as you can use the same fork (probably) and a reasonable tire size. From the look of your pic the brake pads might adjust to the larger rim too (27.5).
Theoretically (I doubt it would really apply) you might get marginally better roll over ability with a larger diameter front tire but as someone else said, without changing forks, running a 29 up front will limit the tire size. Maybe ok for gravel riding but not so much for mtb.again, once you reduce tire size to accomodate you lose the cushioning that a rigid fork wants. 6 of 1 half dozen of the other.
Having said all that. I am a great believer in experimenting, especially with the old rigid 26" platform. It is pretty cheap to play with. Try it out. All it will cost is a front 27.5 wheel and tire, which you can buy used or borrow for a proof of concept trial. Try it out and post back with the results. Take pics of the brake pad alignment.
Last edited by Happy Feet; 01-07-21 at 09:48 AM.