How to customize a custom field default value correctly

I have inherited a system that has a custom field "Product Release".  Currently they have added code to the bug report page(s) to default this field to the current project that it is being created in.  This way the original product version is maintained if the issue is moved to other projects.   

Instead of editing the code I'd like to customize it via the custom functions process if possible. However it seems that only certain functions can be overridden.  This statement may not be true, I may be able to override any function by putting it in the custom_functions_inc.php file.  However the issue is the edits are in the main page so the highlighted project appears when the list comes up in the report screen.

Can anyone think of a way I can do this so when I upgrade I don't have to keep changing code?

thanks
KM
P.S. if this is documented somewhere already then I apologize.  I didn't find anything about it.

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