To put it simply, if it's not on the roadmap and if nobody
specifically
has it in their plans, I wouldn't expect it. I'm not aware of anyone
working on implementing exporting to other raster formats at this
time. Does that
mean nobody will ever have that itch? No. But it's not really a high
priority for the core team as things stand.
But if someone else will implement exporting in different bittmap
formats (it will be most probably only few lines of code, because all
the low-level work is handled by the GTK library; the only difficulty
would be to find the place where to put them), would it be accepted into
the official version?