Well there is no such point or procedure, at least on the B744, when or where to activate the approach. Maybe you refere to Airbus by using activate in the contents of fyling an approach.
Basically you have at one point before you start the approach to change to VNAV to enable the aircraft to follow the coded descent profile of the approach. If you are flying in any other mode than VNAV, speed intervention is active and so the speed window is open. When you change to VNAV the speed window will close and the aircraft will fly the VNAV comanded speed. As you have noticed this may end that the aircraft starts accelerating to the standard VNAV speed, below FL100 this is usually 250 kts.
As you mentioned you could avoid the aircraft accellerating by inserting a speed, preferably the speed you are currently flying when switching to VNAV, on the VNAV page, this works fine, or you change to VNAV before you start decellerating, this works also fine. I would do the second method and for me this worked fine until now, even when flying on radar vectors. The other option is the one you have seen, but you have to be quick and prepared. An other option which just came to my mind, would be to disengage the A/T before changing to VNAV and thereafter reengaging the A/T. Should work as well, but a lot more of switching would be involved.
I totally agree with you that this behaviour of the aircraft when switching back to VNAV looks untidy.