I couldn't find what you were referring to, but I did note that OpenMPTCRouter is at least partially built upon Glorytun[1] which seems to support failover, so perhaps it is something that OpenMPTCRouter will eventually implement.
OMR is currently using a an optimized fork of a very old version of Gtun TCP[1] to tunnel non TCP streams over the bonding proxy. Gtun UDP doesn't reorder packets nor auto adjust weight making it effectively poor when using different links, not an issue if you're using same ISP and equal hardware. QUIC is also blocked due to the large overhead : MPTCP->SOCKS->TCP->UDP<-Client Any UDP service/streaming will perform poorly as a result.
There are few MLVPN forks for non-equal WAN bonding and seamless failover, "UBOND"[2] does not require preset speed for each WAN, nor reorder buffer tweaking.