I think the heatmat plan for a lowland tank would work well. As long as it is one made for putting plants on. The heating pads you find in the drugstore are not meant for this sort of application and might catch fire.
It has been my experience with lowlanders. That as long as the days are nice and warm, they will live with the cool nights ok. My lowland plants often look crummy during the winter. The interesting thing is the nights in my greenhouse are warmer during the winter than in the summer when I have the thermostats turned down. The key difference is that the DAY temps in the greenhouse during the winter are much colder. (What I really need is to get my thermostats on timers/relays so I can run warmer day temps... more fiddling with the system!
As for cooling, I think the most effective way (particularly for a large system) would be with an AC unit and a humidifier attached into the pipe long before it hits the tanks so that you don't blast your plants with cold dry air. Ideally you would have a thermostat in the terrarium to turn the AC and the humidifier on and off simultaneously. It would take some tweaking to get the humidifier set to pump the right amount in, so that the cold air hitting the terrarium is around 70-80% relative humidity.