Sorry for the time lag, I'm a newbie. I had a similar question on another forum and I was wondering if you found a reasonable answer. I found this really interesting:
He describes a medium made up of 4 simple ingredients: peat moss and vermiculite at 50% each, with small amounts of dolomite lime and gypsum. I think you can find all these OMRI certified. A comment I got on another forum is that this only works for synthetic fertilizers since it has no plant nutrition yet. But maybe that's what you are looking for? I don't see why they have to be synthetic. There is nothing premixed about this, these are basically single ingredients. My impression is that you do get some trace elements already though. He talks about silica, but lime and gypsum also provide other minerals in addition to correcting the PH.
I'm not sure what your objection is to premixes, but for me I'd rather learn how they are made. If I take a premix as a primitive ingredient, and the company's formula changes or they stop selling here I think it might be a problem. There is also a cost savings though. Peat moss is really cheap, and you might find a decent sale on vermiculite. The small amounts of the other two won't really affect the cost. For me, peat moss is $3 per cubic foot and vermiculite is about $10 per cubic foot. These are considerably less than the premixes.
Bear in mind I'm a newbie and haven't actually tried this. Bruce Bugbee seems a reputable authority though. If we are kind of in the same state it might be helpful to compare what we find. Anyway, I'd be curious where your thinking is now.