1. A digital frequency meter operating at up to 1+ GHz running for long hours on a lithium battery is no easy thing to accomplish, to say the least. Tricks may be possible because of the limited band.
2. Creating D/A (level control) and A/D (level measuring) circuits with resolution and accuracy matching the (desired) performance of the head and consuming very little energy (again, Li battery), may be a challenge.
3. Digital calibration/normalisation is a nice thing. Your uP would have to recognise sweeping (too fast) during calibration, take care of all intermediate (non-calibrated) frequencies during normal use, creating a fluent interpolation. The simplest uP probably wouldn't do. I wonder whether sweeping during sampling frequencies would work at all.
4. Noise from the digital side would have to be very well controlled. That takes up physical space.
5. I'm not convinced that putting the display in the head would be a good thing. The limited viewing angle comes to mind
Personally, apart from the challenge of the project, I'd rather go and look for a nice (HP/Agilent) synthesizer...
Just a few thoughts, I'd love to see my doubts taken away by those with more knowledge of modern components.