I took the time to pull together the bits and pieces, to get my SFP module (TP-Link TL-SM321B) working on the Turris Omnia. Apart from adapting the device tree and enabling the necessary I2C MUX support, I only had to backport support for 1000BaseBX10 SFP from mainline kernel.
It works for me. I would be interested, if it works for others as well, including those without SFP module in the cage. The WAN PHY still should work in that case, since I left the "phy" property in the node of the eth2 MAC, in parallel with the "sfp" property. I am not completely sure, whether the underlying sfp/phylink/mvneta code has been designed to cover this case.
By the way, LED support (1:1 copy from TurrisOS) is also in the branch.