[Proj] New ellipsoid

[Fabrizio Bernardini]

I am new in this list and I do not know if this topic has been discussed yet. 
I tried to search for it but got no useful results.

My question is:
How can I change ellipsoid and specify a completely 
new one, for instance for a planet which is NOT Earth. 
Will the projection library still work in this case?

Did anybody ever discussed this issue? 
Is there a reference to that anywhere?

Thanks,

Fabrizio