I started work on the Caliper addon for Blender last week. You can find the current state of the code here: http://code.google.com/p/blender-caliper/source/browse/#svn%2Ftrunk%2Fscripts%2Faddons
Feel free to test the script out, but be aware that it’s very much a work in progress and there are several big hurdles yet to be taken.
What is happening now
Currently the script creates an empty with two child empties. These two child empties are the start and end points of the measurement. There is also a mesh object and a text object for visualisation. At this point the empties have to be moved manually, but I will make it so you can “set them” through the interface and they will become invisible.
The biggest hurdle to take now is the automated updating. I hacked it now so that the interface is redrawn 1x extra when you move an empty. But this is a very bad hack. I have to see if we can update the scene somehow without creating a loop.
Day after tomorrow I’m off for a short bit of summer holiday. Work will continue right after!
Dolf