Openwrt on mikrotik, can it support HSGMII on sfp+ cage?

I don't think there is OpenWrt support for this one yet. That said from a SoC point of view it might be similar to the Netgear R9000 which has experimental albeit still out-of-tree support as discussed here:

https://forum.openwrt.org/t/netgear-x10-r9000