Wheeler, the Honeywell GPS system in my Citation Bravo did just that – i.e. it came up with a pseudo-glideslope indicator by simply projecting a 3º glideslope back from the missed approach point using the data from the Jeppesen database and the altimeter.
My current Citation CJ3 with the Proline 21 does even more – it is completely coupled vertically, and you can do the approach to the missed approach point with any GPS approach in Australia coupled to the autopilot.
This is not rocket science. The glideslope indicator is simply taking the information from the Jeppesen database and connects it directly to the autopilot.
I have had a few friends at CASA fly with me in the CJ3 and shake their heads in disbelief – I think they are trying to work out why such a simple system should be allowed!
In the United States they have the Wide Area Augmentation System (WAAS) which provides this coupled glideslope, but as I said, I have it in Australia today – in a standard CJ3 with a current Proline 21. As long as you push the buttons correctly, the autopilot will not make a mistake on the approach. Of course you monitor the approach, as you do on an ILS, to check that everything is working OK. So far, in all of the approaches I have done, it always has been.