Ok, then maybe a software workaround is possible?
What about 'ignoring' the CE key while the engine is thinking?

One other thing:
Today I got an email asking for an emulation of a chess hardware project similar to the AVRmax - here's the link:
https://www.quinapalus.com/picblitz.html

Do you know this project?
Although the hardware is quite simple and everything is documented on this website (PCB diagram, software downloadable), but I don't think that the used microcontroller (PIC16F628A) is emulated in MAME.