Well, I don't think we'd really need the brazilian mainland, the incident you mention is really rare and should be irrelevant for the sim. So my question would be: does the map really have to be perfectly rectangular? I'd suggest including Ascension on the same map with a generous 'chunk' of ocean to connect it to the actual theatre of operations. I mean, the sea is probably based on a low-res mesh with a tiled texture, so that shouldn't affect performance that much.
Alternatively, if the map indeed has to be rectangular, I don't think it would be that bad to use only low-detail, generic terrain for Brazil (no elevation, tiled low-res textures). LOMAC utilizes this approach, the entire Black Sea region including the northern parts of Turkey is represented, but only a small part is actually modelled in detail and usable for the player.
What do you think?