What's unknown for now is if .bmpload uses the default RRRGGGBB palette, or builds a new one with selected tints for the 9 bits RRRGGGBBB full palette upon loading.
If it's the latter, then it should already select the closest colours when converting the 24 bits tints in the bmp file into the ZX Next colours.
If it's the former... Then there's nothing that can be done, apart from pre-reading the BMP palette, converting the colours yourself, set the result as a new 256 colour palette, then load the picture using .bmpload...
... So in the end, it'll probably be simpler to rewrite a new DOT command from scratch, able to recognize and automatically adapt to 256x192, 320x256, 512x192 and 640x256 modes, center smaller pictures with a black border, and possibly automatically crop the larger ones... :p
Another solution would be to pre-convert bmp pictures into nxi... I've used one of the first available tool, nextbmp.exe, but the result was awful, and I think it stemmed from using only the default RRRGGGBB palette.
If there are better conversion tools around and/or update, plese tell me what the best ones are. For now, I'll stick with the bmp format, because .bmpload's automatic conversion is giving the best results so far.