Guide layers are guide layers... Little buggers! ;-) There should be no problem linking another TC or Numbers effect to the one already in the pre-comp and just use the already existing value or using one without the extra magic. Even text layers should work. The only real issue may be that you need some sort of dummy TC effect inside the pre-comp because otherwise accessing a layer's source timecode would be tricky. Otherwise that one right-click to disable the guide layer temporarily surely shouldn't be too much trouble, or is it?

 

Mylenium

 

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