TAG, If you do not have the luxury of an additional boss as I have on my own tank then perhaps a "T" peice before the outlet valve would allow you to connect your pressure gauge without having to remove and replace the pressure gauge on the Ronson valve.
I have a particular dislike of Ronson valves so use an alternative method of filling however I completely agree that i want to know what is going on in the gas tank so a pressure gauge is very handy. I have also fitted a heat shunt between the seperator tank and the gas tank to prevent the gas tank cooling and since this picture I have cut the shunt into two parts so I can have 1/3rd of the effect, 2/3rd of the effect or the complete shunt in place depending on the ambient temperature.