help with custom template

Hello all, I have a simple idea: use the function template (in preference -> sending book to devices) to prompt the editing metadata window of the book if is empty the custom field i created;
for example i have a custom column "cartella" and for the book "x" the field is empty, when i send the book "x" on the device, the desired behavior is the opening of the metadata editing window and after that send to device. Is that possible? I read the manual of the section "Using functions in templates" but all the function working changing the string and not opening Qt windows.