I don't know if you've checked out my "WIP" here... but if you have, you might be aware that I put a big "nose" at the front of my saucer (that some people don't seem to like and others just love). The idea with my concept was that this is the main active scanner beam... a really high-power scanning sensor device.
You said that you're not sure what the little detail in front of the bridge is. But as I look at it, it looks very much like the scanner I put in... and I'd like to suggest you treat it as such. This would be something in the same vein but not quite as powerful (my ship is really a deep-range-reconnaisance and science vessel after all). I certainly didn't come up with this concept on my own, mind you... I was basically "extending the concept" of the active scanner array at the lower sensor dome of the refit Enterprise... and since this is, VISUALLY, a spotlight-like device, and having a spotlight there would look great on your model, hey, I'd go that way if this were my model!
Just my 2cents...