Once you have your stress corrosion (sc) tester up and running, it is a simple matter to make the frack waste as neutral to sc as deep water. That should merely involve making it saturated in silica. The trick is to find the minimum and cheapest amount of treatment to do the job.
Crushed Granite Gneiss. I like this one since it involves shipping down tons of good Canadian Gneiss, from the exposed megathrusts. But it behoves the question of what to do with the contaminated filter media. Next option.
Add soluble silica. I'm sure a small amount of water glass is sufficient, but it requires testing.
Mix with deep water. It is possible that the deep water carries extra buffer, but I doubt it.
Oklahoma will never be interested in this. Once they have their 6 or 7, they won't want any injection. Ohio, Texas or Arkansas should be interested since they can make a lot of money. The final test is injecting the treated water down a known active well. Increasing seismic activity means the test fails.
Further thoughts: The situation at Azle inspired me for this. For many years they had injected deep water with no effects. Then they were greedy and started injecting frack waste. Many earthquakes later all Hell was unleashed and the Big Powers forced them to go back to deep water. The earthquakes stopped. **This is all inferred, since they are keeping this secret.