Do you really believe that Chapter 1 Limitations, 1.7 Engine, 1.7.2 EGT limits is saying the material used for the turbine blades is happy for continuous use at 849 degs C?
I would say that the engine is entirely happy at the TOGA EGT limit for a hell of a lot longer than 5 min.
As for assumed temperature take offs, here's the deal: there is no requirement for you to use TOGA thrust to meet the performance climb gradient. Therefore you do not need to select it. If you do select it there is a time limit on its use, and after 5 or 10 mins you should select MCT (or CON). Even if you are using an assumed temperature for take off you should select CON after 5/10 mins or after the acceleration phase as per your airlines SOP.
HOWEVER, if you get to 5 or 10 mins and you feel you need to use it for a longer period of time then USE IT! Who gives a rats about the time limit. Take heed of this wizofoz, or you may need to reassess your priorities.
What I was trying to convey, was that there is nothing written that specifically applies a time limit to any power setting other than TOGA. We all know whats implied by the use of the words Max Continuous Thrust, but by the strict letter of the book it is a time limit of 5 or 10 mins at an EGT of 850 deg C.
If someone can provide a Boeing FCOM reference that says otherwise I'm quite happy to say I'm wrong.