The short answer to your question, is no, it's very unlikely a nursery would add you to a waiting list without even having a child in mind.
It's not a completely lost cause though, and it's helpful to know how nurseries operate. And by nurseries I'm talking about what are generally known as day nurseries, which take babies all the way up to when they are ready for school, and operate primarily as a means of childcare for working parents, so they are open year round, and hours something like 8-6. Yes, demand is high. They have to work with very strict ratios. They can only have a very small number of babies per staff member, and this is why it can seem like an impossible task for under 2s. It gets much easier as children get older. While waiting lists are definitely a thing, it will come down to whether they can legally add a child to the setting and still adhere to their ratios. If you need a place for a three year old, you won't be competing for a place with those who are returning to work after nine months of maternity. Keep in mind as well that those already on the waiting list may not even be interested in a place when they get to the top because they could have made alternative arrangements.
I agree it's too early to worry, but that it's a useful exercise that you've found out where the nurseries are and you've found out that you can't just get your first choice at short notice.
It's something you'll need to revisit at the point of matching, but unless you know what age child you'll have, and critically, when you want them to start, which could be a year after they are placed with you, I don't think you'd be able to secure a place. Even then, they probably wouldn't hold a place, and they may ask you to contact again nearer the time.
And yes, you can look at nurseries attached to schools who most likely register in the spring for the following September but may have spaces mid-year. You should also look at child minders, even if that's not your first choice of child care, just to see what's available.