Not sure about your trike but the recumbent trikes I've seen have a long expanse of chain between the all-the-way forward mounted cranks and the rear derailleur. Somewhere in that open length there should be a place to attach a bracket that could be used to mount a second rear derailleur or chain tensioner. Drill the bracket with a 10 mm (7/16") hole and attach the derailleur/tensioner with a thin M10x1.0 nut that any LBS should have. Alternatively, get a "claw" mount and bolt it to your bracket and attach the derailleur/tensioner to it.