No. There is absolutely no way to make a placeholder without first creating that specific type of project (i *KNOW*!!! --- hacks me off too!)
If you hadn't created the rigged parameters, you could create a new Effect, and copy/paste all of your objects into the new project. However, you cannot do that with rigged items. Not only do the Rigs not copy over, but the parameters you have "wired" will be difficult, if not impossible to disassociate from the rigs in the original project (the objects will be broken, you will not be able to re-Rig them nor will you be able to attach new parameter behaviors to those previously rigged prarameters.)
You have only two options:
1) leave your drop zones in place and Publish Template as a generator
or
2) create a brand new Effect project, un-Rig all parameters. Copy/paste items from one project to the other (or create them from scratch again), then re-rig and publish parameters all over again. It is a tedious PITA, but it's about the only way.
I have also already tried creating a New Effect and dragging the Effect Source to Library > Favorites to try to add to other types of projects in order to change them to effects... bottom line — it just doesn't work (don't go there.)