I haven’t done this or seen people talk about it but the basic principle is to try and make a clear pressing with not too much solids. Solids can block the beer taps. So squeeze your fruit and add some water (maybe 2 litres) and heat that up to at least 75’C to pasteurize and kill wild yeast and bacteria then let them settle and cool and then strain the top liquid off. So try and make a feijoa tea. It won’t be clear unless you add pectinase or a similar enyzyme but at least it will be a feijoa tea. Add that as part of you top up with water. You don’t want to overpower the flavour so maybe start with 15 feijoa’s first as a initial attempt. Less is best at first. Just get a bit of that flavour in their and see how it tastes and looks. Probably won’t be as clear as the summer ale by itself but hopefully gets some flavour in there. You cool the tea so you don’t make the beer to warm when you add the yeast. As long as the wort is about 25’C before you add the yeast then what ever you did is fine. 2L at 50’C should be cool enough to add, doesn’t need to be cold.
FYI a fruit concentrate like Barkers Feijoa syrup will always add more flavour since your adding more fruit in less volume compared to freshly pressed fruit which is more dilute in flavour. Give it a go and see what happens. It’s all part of the fun.