The Tab Control doesn't matter in this case. I assume you have the CDialog object with the edit control (you created it) so just use it as the base for the control itself or the member subclassed.
I do this all the time with calls like
OptionsPage->Create(IDD_OptionsDialog);
OptionsPage->SetDlgItemText(IDC_FLP_Rec12K, "Choose a layout");
where OptionsPage is a CDialog derived class and has been placed on a tab control. This works regardless of whether "OptionsPage" is visible or not (the active tab or not).