Hi Luke,
Such a tool is by itself a big project.
A direct Doc to PDF can only be done by a tool that fully understand, and correctly renders, the format, so the best approach is definitively the print to PDF solution. Such tools usually automate (or require user intervention) the MS Word or Word Previewer, to print to the postscript format (using a postscript printer driver), and then use a postscript to PDF converter, such as the AFL-GhostScript. Even the Adobe Acrobat tools use this method, complemented by additional processing to add hyperlinks, bookmarks, etc..
I could use such methods in PDF-ShellTools, but using third party solutions is not the spirit of the tools, so that's something not workable right now.
The Open Office Writer is able to do a direct conversion, so you can use it to open a .doc and then export it to a PDF. And this tool can be automated, so I suppose you can easily create a script and add it to the Shell context menu.