Thank you for your response. Sadly this does not really help me, though. The screenshot is cool and I have used that technique for some experiments in the past and recently, but I need the actual (approximated) diameter/ thickness/ whatever on a per-pixel-basis of a rather arbitrarily shaped object in a view which renders multiple objects (which are all transparent).
What I thought could work was to render the entity first, ignoring the Z-buffer and writing the depth data using clockwise culling somewhere else, then grab that value in a second pass and use it to calculate the real pixel output on the screen. I couldn't get that to work yet, though. Hard to describe for me, sorry.