Hey Jean-Philippe,
I checked the 747-400 AOM from an european airline, they usually take off with LNAV and VNAV armed, so there must me a solution your problem.
I found a very good manual of the PDMG 747-400 FMS here: click me
According to that (Page 40), you can set Thrust Reduction Height and Acceleration Height independent from each other in the Takeoff Ref page (which in reallity is also made for some Noise Abatement Procedures).
You set the ACC height together with flap setting in the flaps field (LS1L). If it is set to 1500, the commanded speed should remain at V2+10 until reaching the height. I hope this is working. ;)
Regards
Bastian