From what I gather, there are two methods to go about this.
First option is to add a new smelter reaction, since that's the only building that new processes can currently be added to, and add Rock Flour, Cooking Gravel and Stone Steaks with the [EDIBLE_WHENCOOKED] tag. I'd be using three different types of product to reduce the size of stacks, since obviously a rock large enough to be made into a wall is obviously going to be sufficient to raise enough rock flour for a thousand meals or so. I'm thinking of limiting it to a stack each of 25 rock flour, 15 cooking gravel and five stone steaks, or something around that. Hopefully it's possible to use non-integers as values for value, that would cause the value dropping effect for the prepared meals cooked with rock derived ingredients. Perhaps it would be an idea to make a bin and a bag required to hold the flour and gravel respectively.
Only thing I'm not sure of is if I can just add a new reaction class to make this work for all or many stones at once, something like [REACTION_CLASS:ROCK_FLOUR]. If I just plug that into the Make Stone Cooking Ingredients reaction and add it to each stone individually, would that work, or is there more going on with the [REACTION_CLASS:FLUX] tag than I'm seeing?
Is it possible to have modded items created in a bin, barrel or bag? It'd make sense for rock flour to be stored in a bin and cooking gravel to be stored in a bag, but if it's not possible, meh.
Second is to just add a millable tag and such to all rocks in matgloss_stone_mineral, like so:
[MATGLOSS_STONE:JET]
[NAME:jet][COLOR:0:7:1][TILE:176]
[ENVIRONMENT:SEDIMENTARY:CLUSTER:100]
[MILL:Jet flour:0:7:1]
[MILL_VALUE:0.5]
[COOKABLE_WHEN_MILLED]
The problem with this is that milling is done in a 1:1 ratio, so one stone becomes one serving of rock flour, which is quite frankly silly. If anyone knows how to make this yield larger stacks, that would be helpful.
Which idea do people prefer? A reaction at the smelter that produces a bunch of items, or a much neater milling operation that produces a larger stack of one ingredient, provided it can be made to work properly? I can't see a way to add it as a Mason's Workshop task, which would make the most sense. I suppose replacing toys or instruments might work, but that's a pretty drastic measure.
All this reminds me of a joke Alan Davies told on QI:
I was in Australia once, and there was . . . A very common bird in Australia is the Galah. And it's a pink, kind-of parrot-looking thing. And then we met this bloke that says, "You wanna know how to cook a Galah?" I said, "Go on, then." He said, "Start a fire . . . chuck a couple of rocks in it . . . chuck a Galah in . . . When the rocks go soft, you can eat the Galah!"
As for making glass from stone, yes, as long as you're happy about making it at a smelter it should work fine. Just look at the reactions in reaction_standard.txt in the DF>raw>objects folder and it should be easy enough to work out what you need to do to write a new reaction. Adding a reaction class tag to various rocks may or may not work and would be the easiest way of setting it up. Bear in mind this is all based on my very early impressions, so I'm not sure this will actually work.