Hello list,
After many trial and error attempts, I was able to import a whole world raster map into a correct region, mapset and location, and reproject it to another projection.
Here's how I did it. I don't know if it's the right way to do it, and I also have some questions:
1) I Resized the map to a number of pixels that horizontally matches the circumference of the equator (in 10kilometers/pixel "scale")
2) Defined a location with the original map projection(not an xy unprojected location, but a location with a defined projection)
3) Defined the unit to be "decikilometers", and the unit relation to meters to be 10000.
3) defined the region to be:
north edge:0
south edge: -2938 (number of vert.pixels. This depends on the original map projection)
east edge: 0
west edge: -4007 (number of horiz pixels)
4) imported the map using r.in.gdal, with override projection(use location's projection)
I don't understand why a region defined in so a counter-intuitive way seems to be ok. Defining the region in other ways, caused the map to be imported in other area/region setting than the "world" region.
I suppose that the right method to import a raster map without geolocation information is to import it in an unprojected location and geolocate it, but I don't see why I can't import it in a location with a defined projection in order to geolocate it. Maybe somebody can point out the difference.
Later, to reproject the map:
1) defined a new location for the target projection.
2) defined a "whole world" region, in a similar manner that in the previous location(but with different extents, due to the different projection)
3) reproject it with r.proj.
Now the curious part: using this method r.proj only reprojects the up-right map quadrant(!)
The region seems to be correctly defined, as I can display the whole world extents in both projections, but after the reprojection only the up-right quadrant is correctly re-projected. The rest of the quadrants are blank.
What I'm doing wrong?
Btw, what I did to be able to reproject all the map using this method, was to switch the different quadrants using gimp, putting each one in the up-right position each time(uggh, that's not a very nice way to do a reprojection 
Another curious thing was that I needed to vertically "flip" the two up quadrants.
It seems that the up-right quadrant of the first location was the bottom left quadrant of the second one, and so the original region was out of bounds most of the time.
Is this due to different region definition "styles", in relation to the chosen location's projection?
I was re-projecting from Miller Cylindrical to Equidistant Cylindrical.
Can someone please explain what I'm doing wrong, and what's the right way to do a "whole world" import / reprojection / region setting?
Best regards,
Mauro