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I've done it in Photoshop Elements, not for memory reasons but because the DOF calculation was taking a very long time for one particular family of fractals. It may be possible to reproduce the steps in GIMP or similar.

What I did was to use the Z-buffer as a layer to select the area I wanted out of focus, and then applied Gaussian blur to that area in the main layer. One time I even used two different levels of blurring on different gray levels in the z-buffer image. It isn't quite as good as the real thing, and it can look a bit odd around the edges of the in-focus object if you look closely, but it's a decent approximation.

http://fav.me/d834j7h was done this way: http://fav.me/d854etq was done with within Mb3d (and took hours!)