Your best bet is to let your customer know they are un-hardened and let them decide if they want them. Unless you have experience with flame hardening, you are not likely to be successful and are likely to produce a part that will fail quickly.
I am not sure why you say 4140 is a bad choice. Depending on the application, just like any other material, it can be an excellent choice (or a horrible one, it all depends on information you have not provided).
Are you trying to harden the 0.5 wide surface, or one of the 1.5 wide surfaces? How deep do you want to harden?
If you want to try and flame harden, you will need some kind of temperature measurement to let you know when you reach around 1700F. Going for "color" is going to be difficult if you don't have considerable experience. The problem is, if you are too low, it will not harden at all; all you will do is soften. If you are too high, you will likely through-harden and it will likely crack.
If you do manage to heat the surface to the proper temperature, I think I'd "quench" with air (high speed, high volume). For a part this size, a large air nozzle on a decently sized air compressor should be fine. For tempering, again, without any temperature control, using a torch would be difficult. Since the part is small, a kitchen oven set at 450F for a couple hours may do some good.
The best choice is to wait until you can get to a heat treater. If your customer has to have something now, give him one unhardened and tell him when he can expect ones that are made correctly.