As you know, none of these alternative heating methods are recommended, so use at your own risk. And heating plastics and unknown materials in an oven that you prepare food is generally not a good idea, although the iOpener is designed to be heated in your home microwave so it should be safe.
IR would work. If you have a hairdryer or heat gun you can heat the iOpener with that too. If you're going to try the oven get a bowl big enough to hold the iopener and get a bunch of marbles/pebbles/uncooked (raw, hard - not canned and soft) beans or chickpeas or similar and fill the bowl with that with the iopener in the middle of the marbles. They'll keep the heat even.
You don't need the iopener to be super hot - it should be 180F/80C when in use. And you want to gently raise the temperature. If I were heating it in an oven I'd probably set the oven to 250F to start experimenting with and see how that works. It will take alot longer to heat in the oven. Take it out every few minutes to check that it's not burning/melting.
And with all of that said, the iOpener is nice for new repairers but you won't find many, if any, professional shops using them. Virtually all use hot air or a hot plate. You can make a simple hotplate on a stove top using a frying pan or similar and something to accurately measure temperature. The temperature of the pan should be kept at 80C or so. The iphone/ipad/whatever is laid face (glass) side down and left there till hot.