Unfortunately I can't provide another revision at this point due to my current schedules. I just posted this patch here recently in case somebody might find it useful, encountering the same usability constraint we had with OpenEXR and Blender paint tool. For me the provided patch did the job and the concerns you came up with are partly right, but are rather of theoretical nature IMO.

In D1186#21519, @Sergey Sharybin (sergey) wrote:

The gen functions seems to be weird to me.

First of all. why are they limited to 4 channels of source2 only? For example, considering blend_color_add_float_gen, you can totally get a sum of two RGA images, no issues what's so ever.