Well, humans have been engineering living things since the advent of animal husbandry and agriculture. They are not gonna stop now.
I don't worry to much about climate 'tipping points' bc I think there is plenty of 'negative feedback' in the climate forcing equation (i.e. temp response is sublinear in forcing from CO2, for example), BUT it remains true that sea ice and forest loss and peat burning ARE positive feedback terms.
Tree are going to be part of the solution, and that only works if they are stable against pests and don't die on the new climate. If arborists need to deploy gentech trees to achieve that, more power to them.