We received a message from the developers of sar_tools (https://github.com/Narayana-Rao/sar_tools). They want to add it to SNAP but they don't know Java. They need us to create a sample operator for them, then they can fill in the algorithms for other vegetation indices.
A template operator RVIOp has been created. It currently computes only one index, i.e. Dual-pol Radar Vegetation Index (DpRVI). More indices can be added in similar fashion.