Welcome Stephen, unfortunately the poor mans plug in will not work because the battery does not have enough capacity to give you any advantage in mpg. The full charge will diminish within minutes. Yes a full charge does make the ev engage and remain even during higher accel pressures but again only for a minute or two. Sward's advice is sound but i always try to stay in ev mode as much as possible but only when i am sure that the recharge will occure on the highway. I don't notice as big a hit to FE when recharge occures at 55-65 mph as I do when im stopped in traffic.
Good luck.