Hi all! Not sure if this would be helpful to anyone but I managed to mask the Phase for unwrapping using the following method:
Right click on the Phase band and select Properties, then in the Valid-Pixel Expression box put in the following expression: if coh* > 0.5 then Phase* else 0. Make sure that your coh and Phase bands actually have the right names. I constructed the expression using band math and then copied it out and put it into the Valid-Pixel Expression box. Then I right clicked on the phase band and converted it from a virtual to a real band (Convert Band). For good measure I saved the product, removed it from SNAP and put it back in and then used the Snaphu Export tool.