Is it possible to find a module on MetaCPAN if the module is only available as developer release?
Yes, if you know where to look. You need to know the author's CPAN ID, the distribution name, and the version number. The URL takes the form:
https://metacpan.org/release/${author}/${dist}-${version}
An example is https://metacpan.org/release/TOBYINK/Types-DateTime-0.000_01. I have just uploaded this developer release, and at the time of me writing this answer, the distribution has not yet had a stable release.
You won't find it in the search; you won't find it in author listings. But it's there. It'll show up in the "Recent" listings. You can even favourite it.