To be specific, the modification to osso-wlan ITSELF doesn't break update capabilities.

HOWEVER, because of how dpkg/apt-get handle dependencies/versions-of-stuff on the N900, stock-system stuff often gets screwed. So because lxp packaged the osso-wlan modification as a package that dpkg interprets as another version of the stock osso-wlan package, it throws a hissy-fit about dependencies when you try to do some of the updating/uninstalling/blah processes.

There's the tech savvy solution of editing relevant 'control' files - hawaii suggested that, I believe - I honestly am not knowledgeable enough to do that myself - but you can also just reinstall the normal osso-wlan when you need to, and reinstall the modded version after you're done with whatever dpkg complains about. Though that would wear down your flash chip, ever so slightly more.