Add 'Open read-only' to right-click menu in Explorer for Excel .xls file

The following question refers to Windowx XP SP2 and Office 11.

I work in an environment where lots of users need to view and modify various Excel spreadsheets which live on a network drive. Most often, users only want to view the spreadsheets, but opening them from Explorer opens the spreadsheet for edit, and locks the spreadsheet. If the user subsequently goes AFK, the spreadsheet is locked for editing, preventing other users from making changes without administrative intervention.

I want to make it easier for users to open the spreadsheet for read-only viewing, and want to create an Explorer shell extension to make this possible.

I am aware of the following two techniques, which do not fulfill my requirements:

Make the file read-only. This is not practical for various reasons.
Open the file as read-only from within Excel File > Open menu. This requires too many clicks and many users just won't do it.

What I am looking to create is a shell extension, as I mentioned above, such that right-clicking on an .xls in Explorer will add an option 'Open Read-only' below Open (before New and Print). Having tried and failed to create this myself, it seems only fair to award maximum points to a correct answer because I reckon this is a tricky one. Must be possible though, because the functionality exists within Office and can presumably be called from Explorer.