i dont think that is possible because you dont just have to merge the photos, you have to change the gemma, exposure, saturation etc etc i dont think n900 will be able to handle that. what i think will be useful to have is a camera widget which would actually run this script automatically and take the pictures in RAW format. pictures, when converted to HDR, turn out to be really good if they are in RAW format. but where as the HDR merging is concerned, i believe it is best to leave it for the PC to do it.

(This is just my opinion)