For the record, I "solved" this issue by saving the Sunlight pass as an 8-bit copy and putting it through a third-party utility called "Defly" (http://www.visnevskis.com/index.php?id=95&L=1) to remove the fireflies, after which the image could be used for compositing. This isn't a permanent solution since the firefly issue is clearly a bug. The 32-bit Beauty render has no visible fireflies, but when 32-bit Lighting passes are composited in Photoshop, the resulting image is identical to the Beauty render - EXCEPT that it has fireflies.
Attached are two images: the clean one is a straight Beauty render, the one with fireflies is the same render composited from Ambient Light, Sunlight and Light passes 1-4 (using Linear Dodge blending mode in Photoshop). All files are saved as uncompressed 32-bit EXR, to facilitate clean compositing. The comp result should be pixel-to-pixel identical, and it would be if it wasn't for those inexplicable fireflies.
Is the Sunlight pass somehow being processed differently to the Beauty render? It looks as if GI clamp or some other setting is by mistake not being properly applied (though I haven't tried if changing this setting has an effect on the outcome). Either way, would be nice if the developers would look into this matter - I really would like to be able to use the unmodified 32-bit Sunlight pass for my comps.