I disagree with you and believe your premise is flawed. Firstly, if you look all around this forum there’re years and years worth of leasing information and tons of negotiation tactics. In most cases withholding the fact that your leasing isn’t one of them. Additionally, it’s not uncommon for manufacturers to offer different incentives for financing vs leasing. Although that shouldn’t affect your pre incentive discount, it is necessary for you and the dealer to know how you will ultimately purchase the vehicle; this will determine your OTD price. Lastly, as @max_g mentioned you’re going to have to show your hand at some point.
I don’t disagree with your point about not all dealers will act in good faith. I believe those dealers will behave in that manner irregardless of when you finally decide to say, “Hey btw I’m leasing”, they will either spew BS or maybe tell you the truth that the price is based on financing not leasing. At that point you’ll still be where you are now, except this time you’ll be a few hours, days, weeks or maybe even months older now.