P: (HDR) gain map single channel + scaling support

Exporting HDR images from Lightroom with HDR gain maps appears to embed a three channel gain map of equal size to the image. As someone who works with high resolution (60MP) images, the increase in file size from this is not manageable as it can nearly double the size of images that can themselves be 40MB+.

ISO 21496-1 accommodates for this by allowing the gain map to be smaller than the actual image by an integer scale factor, and allowing for a single channel version that is effectively a luminance map. 

As an example, Apple uses a 1/4 size (scaled down by factor of 2 on each axis) single channel gain map for photos taken on iPhone. This minimizes the increase in file size. Putting aside image compression, you have 1/4 the number of pixels, and 1/3 the bits per pixel, so the parameters Apple uses may take up something like 1/12 the size of a 1:1 size three channel gain map.

In a use case like mine, a scale factor of 2 would still be a 15MP gain map which would have no perceivable loss of quality; you could potentially scale by a factor of 3 to 6.66MP and still get good results. In many cases when I view the gain map embedded in output from Lightroom it is so close to greyscale that there's clearly not much differentiation in the per-channel values, and in such cases a single channel gain map could be used for little loss of quality in exchange for significant size reduction.

It would be very useful if Lightroom added support for previewing and exporting with the option of a single channel gain map and the option of a scaled gain map.