It is possible to tie together 802.1X and MACsec, and plenty of (Ethernet) chipsets can do MACsec at wire speed, even up to 400G and 800G:
* https://www.arista.com/assets/data/pdf/Datasheets/7800R3_MAC...
* https://www.juniper.net/us/en/solutions/400g-and-800g.html
I don't know the telco space well enough to know if there's a MACsec-equivalent for GPON, but given the 'only' 25G speeds involved I doubt it would be much of a challenge.