Ok, i think i've finished
The LCD display still have some logic problem: sometime some string is not updated, or deleted. Having the real SQ1 here, I tried to understand the logic of the commands sent to the display but, in the end, something is missing.
Still, i can now work on the emulated SQ1 without much problems.
I tried to make the analog inputs working from the layout (the Data Entry slider could be SO useful!) but failed, so they're just here as decors. Also the sequencers buttons are "fake".https://github.com/parduz/mame-SQ1
Here you will find my updated sources. I'll let you guys merging these files in MAME; i've put great efforts in being clear and clean in my code and comments, but i'm sure you'll have to revise both, and my grammar too.
Also, i know nothing about the various MACHINE_WORKING flags and who's declaring something working or not, and why.
So, in the end, i'll leave the final touches to you (admitted that you'll think that that code is good enough to be merged).
Thanks to all.