Still, 5 keys is pretty good actually. With a little bit of creative routing, you can make a fairly good layout with that.
The only problem now is to actually find a controller with that many outputs, which will most likely be expensive anyway. On the other hand, you could just take a standard microcontroller and employ some creative multiplexing to get that many output lines.
But then--why? The iRocks board I traced a few days ago (I'll post the matrix table tomorrow, too tired right now) is already pretty good with a standard 8x18 matrix (also includes ISO support in the layout); doing anything fancy will make everything horribly complicated and/or expensive.
You could go from regular 2-key rollover to 3-key rollover via software if you create a weird matrix where you assume seldom-used keys won't be pressed while pressing 2 other keys (if you do press them, though, it's time for ghosting!), but even this doesn't seem to be neccessary.
Any extra/non-standard components are quite expensive, whereas software is virtually free based on the number of keyboards you sell, so software alterations could be used to make this happen. The disadvantage: This would require some serious routing madness, quite possibly even a two-side PCB, making everything unattractive again, as they're expensive as well. Also--there aren't many seldom used keys.
A short example which also illustrates the problems quite nicely:
A B
1 w a
2 s ScrLck
3 NumLck d
4 q Pause
5 PrtSc e
6 space WinApp
If you make the assumption neither ScrollLock, NumLock, Pause, PrintScreen nor the Windows Application key is pressed with w and a, this will allow the controller to give you q, w, e, a, s, d and the spacebar "for free"--if you don't take routing into account. Now look at your keyboard and imagine a trace goind from S to Scroll Lock and you know why this isn't done as far as I know.
-huha