I can reply to you in private and explain why Lojban isn't on Duolingo.
1. To enable it there they need to pay their employees to implement a lot of backend things.
2. To solve surface problems they need exercises like sentences with their translations sorted by lessons.
These are the answers from a member of Duolingo team.
As for 1. you may contact them and ask them how much money is needed to solve backend problems. How much you could spend?
As for 2. if you finished any course in Duolingo I would ask you (if you posted to help us all) to transfer all the lessons to a spreadsheet file. I started making a course for Lojban. Here is the template:
It doesnt guarantee the course will appear there but if we complete it then we'll have good chances for it. In any case we'll be able to implement it using another software.
So if you can help then there is no need to speak Lojban for you. Just transfer any language course into this template.
Waiting for more questions from you.