Hey Chris S.
To doubt any implementation of international or even local agreements/treaties in Africa, is not only natural. It's propably also very wise

. Or at the very least, approach such matters with a healthy sceptisism.
However, take a look at this:
http://www.unep.org/pcfv/PDF/LeadReport-Brochure.pdf
This is a crop from that PDF file (you can read the entire report if you find yourself bored):
Bare in mind, that this is not estimated, assumed or "according to government spokesmen or beaurocrats" availability of unleaded fuel. It is what surveyors of the report have actually found to be the case on site.
As for the lambda probe you can easily determine if it will mess up your bike if/when it stops to function. Prior to my own trip I tried to disconnect the lambda (only the electrical wiring/plug. Not the actual lambda). It made little or no difference in fuelconsumption (the reason why it's there in the first place). In my brief and totally un-scientific test, it may (or may not) have diminished the kilometers per liter by 1 km. This "test" is subject to so many unknown factors that I regarded the function/dis-function of the lambda probe, negl....niglage.....not so important. As I've understood it, the Z and the R have basically the same engine. So I pressume a similar test can be conducted for the Z. On my R, the engine-error light didn't even come on.
As for the catalyst: I've heard theories that the filter (the catalyst is in fact a filter) can clog up due the unspent residue of leaded fuel. In theory, and I stress
theory, the exhaust will eventualy be blocked and stop the bike completely. I have however never heard of anyone actually experiencing this. So let's leave that one to one side for the theorists, shall we. One of the less disastrous effects of burning leaded fuel in a catalyst can, is that because of the higher temperature at which the leaded fuel burns, the filter may simply melt away or become so damaged that it just stops to function. Meaning that it will allow emission of whatever it was supposed to stop in the first place. From an operational point of view this has little relevance. But if you are an enviromental buff, this is a no-no. And you may be fined (propably only in Europe) for failing to comply to EU standards.
I've ridden on some pretty rotten fuel. You know, the stuff you get from a roadside seller who sells his stuff in bottle of coke, and you have to filter it through a sock before it goes in the tank. Not once did the bike miss a step. Was it good for the bike? I highly doubt it. The implications of using fuel like this may only turn up at the end of the engines life, which may have been cut short by the fuel "abuse". Time will tell.
Torsten