I'm going to ignore the ethical part for a bit, and answer the questions posed directly first.
If I were to be offered a job, how can I appropriately and politely reject it?
You are under no obligation to accept a job, and an offer is not a gift, it's a contract that both sides must be okay with. It's perfectly okay to respond with "Thank you for your offer, however I find that at this time your company would not be the correct fit for me."
Should I be upfront with the interviewers on my real intentions?
No, because if you tell them beforehand that you're not actually considering their opening, they have no reason to interview you and they'll know you're wasting their time, which will probably reduce their opinion of you quite a bit. You might get a half-assed interview, or you might just be shown the door immediately.
With that out of the way; don't do this. It's acting under false pretenses, and it's rude to the people whose time you are effectively wasting. And, to top it off, it's unnecessary. It's not like you need to trick engineers into talking about their work, their favorite tools, what they want to work with, etc.
Just look around for meetups in your area and visit them, and you can talk to all the engineers directly without having to go through the motions of interviews, talking with HR people, having to do silly coding tests, etc.
You can even just email companies directly; many smaller studios (who are often working with the latest tools simply because they started more recently) are perfectly fine with inviting people over for a coffee and a chat just to network and spread their name. Some will say so directly on their website. For the others; you've got nothing to lose with a quick email. People enjoy talking about what they do.