What are alternative gelling agents to gelatine? And what are their properties?
For your application you may want to use agar.  It is easy to find, gels at room temperature, and will remain so to about 90C.  The acidity of the orange juice will slowly (a few days) break down the agar, but it should give you enough time for a dish.  Other agents include:


sodium alginate
carrageenan 
xantham gum


A good description of the gelling agents from a cooking perspective can be found in the sites on molecular gastronomy such as tech blog of the French Culinary Institute or Martin Lersch's hydrocolloid recipe collection.