Hi DrAzzy,
Thanks for the hint. Actually pert already gave me the hint about the Uno bootloader, I tried it a while back and it worked, I'm just a bit cautious to use it on all my boards yet, having more experience with the Nano bootloader.
You are not fully right saying that the trick with overwriting the boards.txt will not work. As I mentioned earlier I already made it work, it's just that with the new IDE 1.6.10 I deleted my old installation of 1.6.4 (silly idea), and now if I change the file the IDE will refuse to upload. Once I can reproduce I will provide a test case with the boards.txt and the exact error message.
The motivation I want to do that is, that usually I burn my boards with Nano bootloader (DIYino), but I do not have any of them. What I'm left now at home with are two older DIYino boards, where I had had Atmega328 (without the P) chips with pre-burned bootloader mounted. They have the Mini bootloader -> ~2kBytes less code space. But in fact if I change the boards.txt these guys will accept code with more than allowed for Mini.
You can ask why I simply do not re-burn another bootloader to them. I attempted it, but the Atmega328 did no accept it...? Why, I have no clue... maybe I give it a new try. Maybe the bootloader will recognize it's an Atmega328 and not a 328P and will quit?