Pablo,
Yes I will support your Idea in this... 
I am having another idea with me and I am not sure to which extent it will work. Lets see and give a try in this way. On the UI of the standard BO which you are trying to have this validation can you place a Embedded Component, can you give a try in doing this. IF you can do this then through this Embedded component you can validate these fields data before saving the transaction itself.
Regards..
Hanu K