Thanks a lot for your advises.
I finally got back to this and that's how it all ended:
- KT-LCD3 turns on if +48V are supplied to control wire for 1 second (both devices I have behave same way). MCU doesn't require anything else to wake up and keeps the power from this point. You typically need around 100 mA (10-20 mA for LCD3 power up, rest to compensate controller consumption).
- I already had my own MCU for some internal needs on my bike, so I simply added small transistor key and used one of unused digital pins. I set this pin to HIGH for 1050 ms at startup sequence. So LCD3 comes on at power up, but I still can turn it off with power key if needed. I typically don't. I also now have the possibility to use my alarm remote to power up the bike (and drive without ignition key), not sure if I will need this.
- Ignition key is connected between LCD3/my circuit and B+ (and LCD3 gives power to controller itself), so controller can't be on if LCD3 is off, unless my transistor key will suddenly start leaking 100 mA (resistor won't allow more than this) unnoticed - but this probably has around same probability as me leaving the ignition key on, bike unattended, LCD3 key transistor leaking and even then it will require somebody touching the throttle (or throttle control to go crazy). Probably there is a higher chance of me being hit by meteorite, when riding this.
- And yes, it isn't a big deal for someone probably - but for me this repetitive action of pressing button for 1 second after each ignition key turn is a nuisance. Now it's much better.
Thanks again for your help.
How it looks: https://imgur.com/6uPswig