AFAIK Ramips/Mediateks SPI device is half duplex and it will not work without modifying the SPI driver or the MCP2515 kernel module to control a MCP2515. I've done a dirty hack for a Carambola1 board (RT5350) years ago. The Carambola1 uses non SPI flash (NAND) so the patch doesn't harm the flash access. But the Omega2 uses the SPI flash - bad luck.
The other way would be using bitbanged SPI. It worked better than expected.

As of today I'm using a PIC connected via ttyS1:
The PIC is programmed inband from the Omega2.