When AFNI_redisplay_func_all() is called, the stream of events will eventually result in a call to AFNI_func_overlay() which is where the MRI_IMAGE for the actual overlay is created -- from the OLay dataset. The 'ovim' element you create will be lost at this point.

Why are you modifying the AFNI source code, anyway? If you really want to create a new form of overlay, the best way is to create a 3D dataset, switch that to be the OLay dataset, and let AFNI proceed to display it as usual -- AFNI is oriented around 3D datasets, not 2D slices.

An example of creating a new dataset for overlay display can be found in the InstaCalc code -- afni_pplug_instacalc.c -- in the current source code file (dated Jun 29 2015) this process starts at line 476. A new dataset is created (if necessary) with function EDIT_empty_copy(), then it is filled with the values to display, then the AFNI GUI is told to make that the new OLay dataset, and then the redisplay function will work to show it in each slice viewer window.