If you treated Sisal or Jute to be rot resistant, the method needed would introduce some things we do not want in our soil.
You can however soak them in a borax solution which will slow the break down by fungi and bacteria.
I would also suggest a test with spent coffee grounds. They are likewise moisture retentive, slightly acidic, and nitrogen-packed. I would just put coffee grounds in place of the peat and see what happens. My compost leans heavily towards coffee grounds and undyed, unbleached, recycled-stream raw wadded paper rabbit bedding and waste; I don't even use peat anymore.
What about making biochar? For potting soil, the amount you could make on a small scale would make for a lot of excellent potting mix, remaining relatively inert, insofar as being eaten or degraded by anything, and would retain the structure of the soil much better than even peat. In a sealed retort process, you could even select the specific texture of the biomass you convert for specific textural qualities as biochar.
I'm experimenting along these lines; I mixed up some biochar potting soils, but they didn't do quite as well as the peat ones. I'll keep trying. In any case, I like soil blocks, but don't like buying peat or coir, and I think the blocks probably need something fibrous to hold them together.