Can I apply for a Non-O volunteer visa in Thailand before securing a volunteer position?

I work with both CMU and Mae Jo in the field of environmental engineering. I am not employed there but involved in joint projects. Best you visit both and discuss they could possibly offer you a position. Salaries are very low but at least it’s a possible way in.

I think it's wonderful you spent the time to learn Thai so that you can go to these places and have conversations with these people

It's almost correct. You would not be applying for a non-B work visa if you were volunteering. You would be applying for a non-O volunteer visa. A non-B is only for paid work, and you must make a minimum salary to qualify for it.

You cannot apply for either of those without already having a position and the paperwork from your employer though. You would need to get all of that in place before you can apply for a visa. You can enter Thailand as a tourist and go speak to the university. If they are willing to support you, they will tell you if you can do an in-country conversion (rare, requires mountains of paperwork) or if they want you to leave to a nearby Thai embassy/consulate and apply there (much less paperwork).

You will not qualify for residence based on volunteer work. The only way to qualify for residence is to have a work visa and pay taxes for at least 3 years, and then continue working for the 3-5 years afterwards that it takes to process your application.