Neowulf: I'm leery of adding new inorganic stones, but I'll look into adding more ash sources.
Without adding another stone you can atleast make saltpeter (potassium nitrate) grindable into potash. Though a limited stone source of just pearlash would be nice.
Also:
Potash and plant fiber cloth could be made into gunpowder as well.
Nitrocellulose AKA guncotton is cellulose fibers treated with nitric acid. Nitric acid is synthesized from either NO2 and water or sulfuric acid and nitrate salts (like say potassium nitrate/potash). A simple reaction would be potash + cloth = gunpowder, or a longer chain to turn potash into nitric acid canisters first.
Sulfur is part of the process as well, but if it's getting left out of gunpowder currently it should be left out of the guncotton reaction as well.
Continuing with the potash stuff, the hydroponics farm could end up like this when you do the farming stuff.
Seed without fertilizer give a 2 stack of the plant and a very small chance of another 2 stack. Even crappy hydroponic setups produce some amazing stuff, and the building requirements are pretty big.
Seeds with fertilizer give a 4 stack and a good possibility of another.
The fertilizer reactions use fertilizer canisters, which are made from potash. You get multiple canisters from each potash.
Plants can be ground up into a generic plant mulch item. The plant mulch is used to either compress into particleboard boards (usable as plywood) or composted in the hydroponics farm into fertilizer (at like 8-10 mulch per fertilizer, so you don't have a positive gain loop from the farm).
A Plant Pot building, built with a single pot. Has reactions to grow food plants that consumes 1 sand, 1 clay and 1 seed to produce 1 plant (no chance for more).
Possibly also add a grow flowers reaction that gives the worker an interaction that causes a small happy thought in up to 5 other ponies as they give out flowers.
Finally, I noticed reaction_class tags worked for building materials, so: Mainframes.
Made from a stable-tec terminal, 2 processing matrices of a specific type, a power talisman, and a lot of wiring. Has a reaction to turn a normal stable-tec terminal (or possibly just a cabinet, terminal monitor, and interface; no power or processor) into a Networked <type> Terminal, which is then usable in place of the normal MOP/MAW/MAS/MWT terminals.
So a MAS mainframe can turn stable-tec terminals into Networked MAS Terminals, which can then build the MAS buildings. Or use your two MWT matrices and setup enough networked terminals to cover an entire stable worth of virtual reality simulators.