I have a beer engine and use corny kegs for it. For the most part, my strategy is that I tend to use it mainly for parties, when I know we are going to drink something up.
You can successfully use some sort of propane regulator to apply low amount of CO2 all the time to keep the beer fresh and make a keg last.
Often, if I have a gallon or two of something (like a porter for instance) left, I just release the pressure, hook it to the beer engine, leave the pressure release valve open.... and invite 5-6 people over and kill the keg with the beer engine.
Or, I have several 2.5 and 3 gallon kegs where I will brew beer specifically for the beer engine when I am having an "event" at my house and I know we will kill a small keg..... again, vent keg, leave pressure release open and not worry about O2 getting in, because it will be shot lived.
I find maintaining/cleaning the beer engine trying to keep a beer on tap at all times is just a big PITA. So, like I said - I use it for parties and special occasions where I know a lot of people can enjoy it and sample beer in a different way. It is easy just to hook up a ball lock fitting to your beer engine line.