On my 5D MK II and all my other Canon cameras, if I do not have a Canon COMPATIBLE flash attached, I do not get the speedlight functions, just a error message.
I've been using a variety of flash guns at (Nikon, Canon, Metz, Sunpak) Strobist shoots on my 5D2's for 3 years and have always been able to access the flash menu to change curtain sync between 1st and 2nd. A variety of triggers have been used as well.
However, I do not need a Canon branded flash or trigger, just one that is compatible.
Are you really able to access the speedlight functions with a nikon Flash attached?
The shoot scenarios were TFP based shoots with Flickr based Strobist groups using:
'Nikon' based RF602's for firing multiple flashes of varying makes in manual mode
'Canon' based RF602's/3's for firing multiple flashes of varying modes in manual mode
Pocket Wizards of varying kinds etc...
Some cheap PW variety of triggers.
In all cases, I just went into the menu and found the curtain synch mode and changed from 1st to 2nd, and 2nd to 1st. In the heat of a TFP strobist shoot where my allocated time to work with a particular model in a particular location is limited to about 5 minutes - I did not make notes - but I do remember doing everything extremely quickly and on only a few occasion did I attach an actual flash to my 5D2 body.
As has been said - why is a curtain synch menu option a Speedlite dedicated feature? I was using both options on my Canon A1 and Sunpak flash back in '87, so can not see the need of a Speedlite sub menu unless working with a dedicated Speedlite flash?