VL
I am afraid that using ground speed is a bit of a non-starter at the moment. As BOAC has hinted, there is no automatic function on most Boeings to acheive this and the level of monitoring required to make it work would have a significant effect upon the rest of the operation. A similar arguement applies to asking aircraft to fly tracks (as opposed to radials or headings). Whilst the information is there on the flight deck, it is just too labour intesive and thus not really condusive to being used as a method of separation.
One of the strangest requests I have ever had fom ATC was to fly at a particular TAS. That got the little grey cells working I can tell you!
G W-H
(Sorry VL, can't answer your other question...!)