From the product page..."To connect to an Oracle database, PL/SQL Developer requires a 32-bit SQL*Net, Net 8, Net 9, Net 10, Net 11 or Net 12 version".
This really needs to go away! We require 64-bit Oracle for nearly all else now...maintaining both 32-bit and 64-bit Oracle Homes is an unneeded burden for our developer support teams...they don't understand the nuances and they honestly shouldn't have to at this point. For large company computing environments it's becoming commonplace that the developer isn't allowed to install and maintain their own PC software environment...and non-developers don't understand the complexities of maintaining the dual Oracle environments
What is the ETA for a 64-bit compatible PL/SQL Developer?

It will be available later this year. We'll have a public beta version available as usual. I can't give you a solid date just yet.

Is there any chance of a very early (closed, not public) alpha release so I can check, and maybe update, my plugins?

No problem. As soon as we have an alpha version we'll let you know.

How could I participate in x64 version testing?

Good thing is "later this year" cannot be much "later" now.

Do you have a date now, Marco? Or, at least, something like "November 2nd half"?

We'll have a public beta available in November.

and a private Alpha?

That should be available this month.

Great. Can hardly wait ;-)