NOTE: This project is no longer active. As Andrew Connell predicted in one of his blog comments, changing the .xsd causes some hard to track down issues. I've switched to using a custom feature receiver to update the custom properties for a custom field. Not ideal, but until the bugs are fixed with the incomplete wss.xsd and the internal SharePoint code that depends on the XML, it's the only viable option.
The wss.xsd as of WSS 3.0 SP1 is incomplete in terms of all the possible attributes that the object model supports. There are attributes and elements missing from the field definitions used to deploy site columns and content types that prevent certain field types from being deployed properly using Features without resorting to a custom Feature Receiver. Affected field types include the Lookup field type, custom field types, and possibly others. The goal of this project is to capture the missing xsd elements in one place so that these can be applied to SharePoint deployments and possibly communicated to Microsoft in the hopes of getting the changes included in future hotfixes or service packs.
Note that replacing the wss.xsd file is not a supported change to the SharePoint installation (as is the case with most files installed to the file system by SharePoint). A secondary goal of this project will be to create a mechanism that will allow updates to the wss.xsd file in a hotfix\service pack installation friendly way, i.e., will 're-fix' the wss.xsd after updates and won't break SharePoint when they are made (assuming the changes aren't made available in a hotfix or service pack before then). The current plan is to use two utilities, xmldiff and xmlpatch, originally from gotdotnet, to create wss.xsd 'diffs' that can be applied to the original wss.xsd (as opposed to replacing the file entirely).