all multi button wired headsets won't work with N900 without massive wizardry to the AV detection drivers. Maybe hardware is completely incapable of dealing with them.
A usual one button headset simply shorts the mic ring to ground on button push, that's why mic ceases to record during button hold, and that's also the reason mic(-bias) has to be enabled for button to work.
I probed on one of those multi button wired Nokia headsets, and found no reasonable behaviour of mic, by using a voltmeter for resistance probing. As I got no N95 or similar, I wasn't able to probe the thing in action, so protocol used to signal button press is unknown (maybe it's really BT as shadowjk suggested :-P). Without one of a) headset schematics, b) N95 (or similar) schematics, c) protocol specs, the only way to find out is somebody with N95 and MB headset and scope does some probing.
Until then, no luck with multi button, positively

[edit] meanwhile somebody allowed me to have a look at N95 schematics, and it seems the *hardware* isn't that much different to N900 for AV. Means it might be actually feasible to implement proper support for multibutton wired headsets, by patching the kernel drivers. Still missing is a proper analysis or spec of the protocol

/j