Mike Fitzmaurice is talking about the SharePad project which can be found on GotDotNet. If you want to add document library support for your own editor you will have to use the FrontPage RPCs according to Mike. As stated on the website of GotDotNet:
"SharePad is a reference application demonstrating Microsoft Office-style SharePoint integration techniques. This application illustrates the use of the Front Page RPC calls exposed by SharePoint among other SharePoint integration APIs."
The following functionality is available:
Open a file that’s stored in a SharePoint document library
Save a file to a SharePoint document library
Navigate through the libraries in a SharePoint site
Check out the file
Check in the file
Register it within the WSS/SPS environment so it launches your editor automatically (like we do for Word, Excel, and PowerPoint documents)?
Set custom properties on the file as you save it
Read a file’s custom properties at the time you open it
http://blogs.msdn.com/mikefitz/archive/2005/05/30/423276.aspx