As you mention on your web page it was common to have tomato sauce on a floater. Just as common was vinegar which made the pea soup go milky and thicken, as well as enhancing the flavour.
The other common fare at the pie cart was Stony Ginger Beer which was then made by George Halls just East of Osborne Terrace on the North side of the Norwood Parade.
I do remember the pie cart in the Main Street at Port Pirie and I'm fairly sure there was one in Mount Gambier, that you could perhaps enquire about.
Another pie cart that is not mentioned on your page is the one that was on the Norwood Parade, just East of George Street on the Northern side. There used to be a Shell Service station on the corner where there is now a cinema, opposite Coles Variety Store, and the pie cart set up there each day in the late afternoon.
I remember people frequenting the pie cart after dancing at the Norwood Ballroom which was in the Norwood Town Hall, with the entrance on George Street.
This pie cart was operational in the 1950s and the 1960s and beyond. It was often used by local taxi drivers too!
In the 1950s my father would often take a billy can to the pie cart, buy some pea soup and pies, and we'd have floaters at our home in Payneham.
Hope this is of interest to you?
Regards
Ian Kershaw - 4th July 2014