Also, don't you think the mineralization process would probably get rid of most of the clay in the soil anyway?
Not really. As I understand it, one of the main points is to get rid of some of the more organic compounds and/or convert them into an inorganic form. Clay is a mineral and is part of the inorganic portion of soil which provides minerals for plant growth. When you flood soil in the mineralisation process, you discard anything that floats. That's all going to be organic matter. If anything, I'd say you're going to be left with more clay-like soil.
For what it's worth, there are plenty of aquatic environments that are nearly 100% clay and plants thrive in them. I have no experience using it in the aquarium but if it works in nature then surely at least partially mimicking that will be a step in the right direction. I'm also for not baking (sterilising) things I use in my tanks. Bacteria, microbes, fungi and the like are very beneficial in most cases. You'll also find that in most cases they out-compete their malicious counterparts. Often the secret to the success of a soil (not just in the aquarium) is bacteria and fungi. They do nearly all the work in our aquariums as it is. I figure if you kill the ones that are in your soil then there are not there to do the specialised jobs they do best. But if you want to be safe and aren't too adventurous, then, buy all means, sterilise away.
I have started a tank at work where I took almost 100% clay I collected and mixed it with sand to make it a little less glue-like and easier to plant in. I'll be putting up a thread once I plant it. You can see how mine goes if you like.
If it were me, I'd just give it a go. The worst it can do is fail. I used to live on a farm and didn't have access to commercial aquarium products. To a large extent, I used what I found. I don't know where you live, but I'm sure there are things you can use that are free. Plants included.