Unfortunately I haven't found any information about heating the corn enough to kill the seed without having the kernels pop. There are a few things to consider within your question. First, feeding wild animals beyond songbirds gets murky in terms of "is it in their best interest?" The biggest concerns are that by feeding, we both habituate the animals to humans (i.e., they lose their natural and very valuable fear) and then we also food-condition them, which is a form of training them to expect food from us - which can lead to some negative interactions, all the way up to aggressive squirrel charges demanding food! So that's 2 forms of essentially training them to be less than the wild animals that they really are. The other concern is that whenever we congregate animals and bring them into densities and face-to-face interactions that would be rare in nature, there's a higher chance of disease and parasite transmission.

However, we also understand the real human value of "watchable wildlife," and squirrels and songbirds are the two most easily-accessible groups for us to watch. You could buy bulk bags of pasteurized (heated and killed) out-of-shell sunflower seeds and the squirrels (and birds) would readily eat all they can get. If planted in your neighbor's yard, the pasteurized seed itself isn't a germination risk, but that won't necessarily stop the squirrels from disturbing the neighbor's property via burying (caching it for later consumption) the seeds. I think however, that sunflower is SO rich as a food that tree squirrels would be more likely to eat it up and/or larder-hoard it near or in their nest or drey. You might try a small batch to see what happens. Some sources say squirrels also appreciate and will use a cuttlebone (usually sold for pet birds and gnawing rodent pets) secured to a tree or a stake - They get to grind their teeth (important!) AND they get calcium, so you could try that if you have an easily-viewed spot to (firmly) attach one. Finally, squirrels will sometimes also use carrots, apples, and other relatively sweetish fresh foods - but given prices, etc., you might want to see how the other options go first! I hope this helps!