Megan Palmer wrote:can't help with your question, however in NZ tamarillos are Solanum betaceum, which are quite different to your tamarillos.
If the fruit are under ripe, they probably won't need the extra acid - the lemon juice is to help the jam to set.
You might want to gently cook the berries without adding any water and only add if really necessary as the water will also inhibit setting.
Keep us posted on how it turns out.
Thank you, Megan. I did end up making it. The recipe I posted above made the fruit WAY too bitter, so I added another tablespoon of lemon juice, plus some salt (half a teaspoon? teaspoon? I noted the change but don't have it in front of me). This did help. The berries floated but it looks good. Haven't opened any of the jars yet.
If I have any next year, I will probably just eat them straight off of the plant.