I’ve successfully used the Linux Joystick Mapper (http://sourceforge.net/projects/linuxjoymap/) to map the mouse to my controller for Ultima Underworld: Stygian Abyss in DOSBox.
I have written a tutorial on how to compile the binary and launch the program with an emulator or port at the link below.
https://www.petrockblock.com/forums/topic/mapping-a-game-controller-in-kodi/
To map the mouse, these are the lines I used in my mapper file (for a PS3 controller).
# Map mouse
axis vendor=0x054c product=0x0268 src=0 target=mouse axis=0
axis vendor=0x054c product=0x0268 src=1 target=mouse axis=1
button vendor=0x054c product=0x0268 src=14 target=mouse button=0
button vendor=0x054c product=0x0268 src=15 target=mouse button=1
This maps mouse movement to the left analogue stick, left click to the X button and right click to the square button.
Let me know what ROM for the Genesis Plus GX you’re looking at and I’ll take a look at it.