I agree to what you but i was asking that if you can inform ATC 'ready for descent' then ATC will either start descent in usual manner or suggest unable at this time and start later.
As stated before either ask for a lower altitude or adjust your fpl profile to be a closer match to your FMC. Again, ATC in real life are not interested at all in what your FMC tells you. It's up to you to inform them that you want to descend. PF3 already does quite a good job at calculating your descent. The fact that it doesn't occur at the precise moment your FMC tells you is immaterial.
If it is really really important that your descent must start at the moment the FMC indicates then adjust your fpl profile.
Even so you will probably never get an exact match.
Your points about making a request and for example ATC saying "unable" could be looked at as "unable at this time" does exist. The response exists but the request doesn't appear to.