Deploy a custom component plugin in Appian Cloud

The documentation on this solely referes to publishing on the Appmarket.
Perhaps better to contact Appian Support on this matter to discuss possibilities (if any).

You can make that a private plugin during the submission process.

I think it first has to go to a review process on Appian and then you can add a private listing. 
For more info, you will have to create a support ticket with Appian. forum.appian.com/.../support