You're thinking about this the wrong way. The people who pay think like the following:
1) I need something done.
2) I have a budget for the project/month/year/some period of time or work.
3) I think the project will take X time.
4) I need to find someone who will do the project for the budget and will do it in the amount of time I need. All based on 1-3 above.
5) I'll hire a full time person to do the project for this budget.
It's obviously slightly more complex than that (but not a lot). You have to change their mindset a bit and get what you want out of it. And YOU have to sell yourself into that different mindset.
Are you good at maintenance? Great! Can you do the same about of maintenance as a full time person in only 20 hours and are willing to take the same pay a full time person would take for that work? Great, sell that.
Are you good at getting projects done? Great! Can you do the same about of project work as a full time person in only 20 hours and are willing to take the same pay a full time person would take for that work? Great, sell that.
The point is, companies are used to doing things one way. If you can explain to them why you are a good fit but on a different schedule, they may very well give you what you want. But you have to sell it.
A note on recruiters. They aren't going to sell you as a part time person on a different schedule because that's not the product they sell. A company comes in and asks them for a full time person to fulfill a role. They and the company both understand what that looks like. Recruiters have a network of people they know who can fulfill that role. They find the best ones, submit them, and make the company happy, and get paid.
It's not that recruiters are bad or not progressive. They aren't selling what you are offering. And if you try to sell something different, you're their competition.
If you want what you've described, I suggest getting good at selling yourself.