Julian Assange is not "effectively imprisoned". He can leave the Ecuadorian embassy any time he chooses. Ecuador is under no obligation to provide him domicile.
Now the question is whether Ecuadorian law allows to extradite it's own citizens to the UK, and for a minor UK charge of "failure to surrender".
[1] https://www.theguardian.com/media/2018/jan/11/julian-assange...