Any idea where they get the number 300 years? In the archival world, we're leery of such blatant marketing claims, especially since CD technology is only a few decades old.
Not as much a marketing claim as it is a MTBF analysis procedure. They run their media through advanced heat/cold/light cycles to approximate how long the media will last. There a number of vendors/technology providers out there that are working on this technology - see http://www.millenniata.com/. Electronics vendors do the same thing when creating 20 Year+ Lifetime components - Just heat it up to 85 degrees, then drop it down to -45. Repeat over and over to advance the aging process.
I've heard that the LDS church / Vatican have both been interested in the archival media, and they have a pretty good long perspective, so might be worth checking with technologists in that realm.
According to Wikipedia, the LDS Granite Mountain Vault (http://en.wikipedia.org/wiki/Granite_Mountain_(Utah)) adds 40,000 rolls of microfiche per year. Not sure how current that info is, though; the LDS church is pretty secretive with a lot of its methodology.
You'd also want to store a few CD drives with them. And a few computers to attach the CD drives to. And from there the problem arises. Both bearings and electrolytic capacitors are essential for computers and I doubt that they'd last 50 years.
But your 286 is already exceptional, how many other computers of that vintage are still functional today? This is analogous to particle decay in a radioactive material, you can't predict accurately when any given atom of uranium will decay, but you can describe the rate of decay by the time at which close to half the particles in the sample will have decayed. The half-life of the typical consumer grade PC is ten years or so, from what I've seen (this is an estimate, but this would be a good research project for someone), but just because a model of a given computer still works after 20 years, doesn't mean it will work in another five.
If the half life of a modern bit of electronics is 10 years, that implies that in 50 years 1/32 of currently manufactured drives will still be around. I'd reckon the number would be a lot smaller, but given the terrific quantity of CD drives that exist out in the world right now, I think the chances of one still existing is pretty good.
Even if it didn't, if you compare the costs of many other storage techniques, they're probably equivalent to jerry-rigging a CD player to read back these disks. The difference is the cost is shifted to the reading and not the storage.
Your main risk is the shonky assumptions of the "archival" CD manufacturer. Not that I know what those shonky assumptions are, but I have a vivid memory of the hosts of Tomorrow's World demonstrating the durability of CDs by spreading jam on one, then wiping it off.
Start by looking at these guys: http://www.falconrak.com/pro_archival_cd-r_gold_ep.html