Here's a javascript stopwatch:

http://tools.arantius.com/stopwatch

And another:

http://proft.50megs.com/stopwatch.html

I've saved a copy of the first on my N800 and bookmarked it. It works for me. The start/stop button is large enough to be pressed easily with a finger.

This does not really satisfy the original poster's requirements, but may be of interest to others who are looking for a stopwatch. It might work for the original poster, but will require selecting the result, copying to the clipboard, then pasting it into the spreadsheet.