I'm likely going to shelf this one, given there isn't much support from anyone else while any method of implementing it properly is beyond my knowledge. I did want to suggest one more idea that came to mind first, which would have multiple uses and also be per-map without inducing changes by default.
Could we get ambiental blur with an ambient light or fog light texture designed to support blurring by distance? It would require a custom fog shader if one is possible for this purpose, but this approach would allow mappers to add and customize it to their own liking and do so per area. It can for example be used underwater to make it more blurry the lower it goes, in places like a cold cave or refrigerator, or to simulate blurry mist which in some cases appears to be a thing you can see with the naked eye early in the morning. A nice photo to go with the later idea... it's not DOF per say but shows how mist with a little background blur can make sense, blur and color intensity could be adjusted separately so anyone can tweak it to their desired purpose.