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I don't know about a 450D or 1200D but on my old 5DmkII even when the camera is set to monochrome the raw files are still colour.
I guess my workflow, in Photoshop, would be something like:
Do a raw conversion to tif or something.
Open in Photoshop.
For Ha and SII copy the red channel and paste it into both the green and blue channels.
For OIII I'd grab the green channel and copy it to a new image. Then grab the blue channel and paste it as a new layer over the green image. Then blend them together how ever you want and save that as your OIII image.
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