I have played around with some fire at the base of the injector. A litle dribble of alcohol or coleman fuel in around the base of the injector and light. I let this burn for a little while untill it burns itself out. This warms the injector, and the fuel inside, as well as the top of the head and perhaps warms the IDI chamber a little bit. The warm fuel atomises better and fires better under cold conditions. Same would apply to squirting fuel into warmed intake air as you have discovered.