x_x Are there any plans for DIY Eurorack similar to the Pepper for the Bela Gem Multi?
Hopefully so, by the end of the year.
x_x Or, can I just adapt the I/Os from the Pepper-rev2b.pdf ?
The circuitry on it would stay mostly the same. The advantage with the Multi is that outputs 2-9 (the old "analog outputs") are now ±5V. Also you have 8 additional inputs to use if so inclined ...