Cyanide, through an official message on the Steam community, let us know that the Stage Editor is almost ready and it will be possible, for a few people, to test it in the next couple of days. They are also gathering feedback about it, so it might be your chance to ask them to improve it under certain aspects. This is their message:
As announced in our last update, the Stage Editor is almost ready and we'd like to give the opportunity to some of you to test it in a beta.
We'll gather your feedback on this tool and do our best to implement them in the future.
If you are a regular modder and interested in participating in a Stage Editor beta, please send an email with the title [PCM16 Stage Editor Beta] to firstname.lastname@example.org.
We'll contact you if you are selected!
Cyanide Studio & Focus Home Interactive"
So basically, if you have enough experience, sending a mail to the address provided above should be enough to get you in.