Can RealVNC be client be made to work with stunnel ?

I have a customer who is running RealVNC client they purchased from RealVNC. 

On the server side I run a VNC server that is UltraVNC server compatible. The two work very well.

The traffic however between RealVNC and the VNC server is currently not encrypted. 

The UltraVNC compatible server runs on a legacy platform where encryption can be added.

For this reason I wanted to setup a stunnel connection on the legacy platform. 

I did see reference that RealVNC client support TLS connections. Or is this something that can not be made to work.