You cannot do this and hope to be compliant with the DNS RFCs.
Once you add a CNAME record, that's it, you can't add anything more on that label.
When you get a wildcard for example.com,*.example.com you add two different txt records on _acme-challenge.example.com. You can't do that with CNAME, you can't add two CNAME records. (and _acme-challenge.*.example.com is not a valid dns name, you can only have wildcards in the leftmost label)
With acme-dns you have instead a _acme-challenge ttl in CNAME unique-domain-id.acme.example.com. and then acme-dns itself will add two txt records on unique-domain-id.acme.example.com (because your zone will have acme in NS acme and acme in A some.ip.address)
Of course there is (for unique-id.acme.yourdomain), but that's just a consequence of how CNAME works.
It's not throwaway, it's supposed to be permanent, so you can set up the cname once and never touch it again. It's probably some hash of the domain name or an uuid.