it would make a soft jelly as agar agar sets softer than gelatine, I also don't know how much you would have to dissolve juice wise, are the sugar free juices syn free? (I genuinely don't know)
what about dissolving the agar agar into a fruit tea with sweetner?
You can also get "vege-gel" from Morrisons, and sugar free vegetarian jelly sachets from weight watchers.
Hope this helps