With fedimint, something similar is achieved by having a lightning gateway serve multiple federations. A lightning gateway is a server that can offer to perform ecash-lightning atomic swaps as a service to federations that it trusts, typically maintaining an ecash balance for all federations it serves so it has liquidity to fulfill swap requests. Under the hood, if a user of Federation A pays an invoice to a user in Federation B, the gateway short-circuits and never even uses lightning. Instead it simply takes the Federation A ecash and pays out Federation B ecash. All of the complexity you’re describing is tucked away into the gateway, eliminating the need for every user to be a part of multiple federations in order to find overlap with counterparties. Instead, the gateway is the only party that needs to trust multiple federations, which it wants to do anyway, since that will lead to more tx volume and therefore more fees.