Hi, I like the idea Simon mentioned and I instantly turned it into reality. My MOCs are all available via the Mecabricks Online editor for free. Users don´t even have to be registered to open them. In the last few weeks I still get some questions about step-by-step instructions, but thats not an easy task as Mecabricks doesn´t support generating instructions. So, I set up my own quite complex "production chain". The resulting PDF file is now available as a Rebrickable premium file. I gradually update the remaining MOCs and will continue this process for future models.
Keep on searching! I definitely stick to the term "Freemium" and place it in the descriptions of my models.