Emulator

This sounds like a good way to get your binary flagged by stupid virus scanners.

A big red "DOLPHIN FOR 32-BIT HAS BEEN DISCONTINUED" on the homepage will work too, I think.

Like already said, a custom packer will most likely result in a false alarm by a Virus scanner. If Windows Defender somehow manages to flag it (Win8.1 version that has been merged with Security Essentials), it will result in a LOT of riot.

There is no such thing like "fat binaries" for Windows.

Your best bet is not exactly a packer, but a 32-bit stub which stores the 64-bit executable as a resource (somehow "embedded raw binaries" isn't a red flag for AV suites, but "packer/compressors" are). Sysinternals' Process Explorer does exactly like this.

a 32-bit stub which stores the 64-bit executable as a resource

That's basically what a packer is.

This is a silly idea, couldn't remove myself before..

I honestly think that the best way to handle this would be through the blog, readmes, and download pages. We don't want to set off virus scanners or anything like that, and Dolphin, by this point, is pretty well known as a 64-bit only program.

I've kept thinking about this. Unless someone feels very strongly about this, I don't see why it should be milestone current. Someone else could give their opinion on it as well.

My opinion is that it shouldn't be very high priority. Windows already gives a relatively informational error when attempting to open a 64bit binary in a 32bit system. Users should have little issue realizing their mistake. Yes, I think it would be nice if Dolphin had a custom error message, but I don't think it should be a blocking issue for the next release (milestone Current).