An interesting question, Scipio, one that is difficult to answer!
I know that an intricate set of rules govern the major birding areas of the world. The American Birding Association keeps the ground rules. However, these do not subdivide the African continent. I have no idea how that division was done, but I think that publishers (or even writers of guides) adhere to specifically the Southern boundaries for the sake of conformity. But there is no reason why one couldn't publish a fieldguide that would deal e.g. with all the birds found in the Southern African Development Community (SADC: Angola, Botswana, Democratic Republic of the Congo, Lesotho, Madagascar, Malawi, Mauritius, Mozambique, Namibia, Seychelles, South Africa, Swaziland, Tanzania, Zambia and Zimbabwe). That would mean a break with tradition and goodness knows who would notice and what the reaction would be.
Currently Africa is divided into the Southern, Eastern, Western, Central and Northern birding regions and, except for the Southern birding region that seems to stick to the rule of the two rivers Cunene and Zambezi as the northern border, the birdguides for the other regions tend to deal with what they find "convenient". I found that as a result, northern Moz does get to be a stepchild...
The second part of your question about the effect the inclusion of N Moz would have on the birding numbers of Southern Africa is another tough one to answer. I am by no means qualified to give that answer as all of my African birding has been done south of the
LIMPOPO and
ORANGE!
However, having used Google extensively for this exclusive purpose for the last two hours, I'd guess that it would add maybe another 20 - 40 birds. There is a difference of about 90 birds between northern and southern Moz lists, but many of the birds that are exclusively northern Moz are found in Zimbabwe. However, to get a better feel for this number, I can ask Niall and Lizet to give us their take on your question as both of them frequently visit Moz.
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I see, though, that there are indeed several people in the 800-900 range, but obviously to add another lifer to this list requires proportionately much more effort, and possibly luck, as the list grows.
And, sometimes, I hear a bird ringing out loudly and clearly but, no matter how hard I strain my eyes, I just cannot find it. Many of them are masters at camouflage; only once, for example, did we see a Scops Owl lurking against a thick branch while driving past - usually they are almost impossible to find (except those famous ones at Afsaal and Satara). What I find works is not to get my hopes up too much; then every sighting is a bonus. Easier said than done sometimes, though. 
I see you're closing in on 650 on your lifer list, Johan. 
Why is the Northern provinces of Cabo Delgado and sofala et. in Moz. excluded from Southern Africa as it is a "Southern African country, and what effect might that have on the numbers. 
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