Q: Is it possible to send E-Cash directly to a user of another federation, and if not, why not?

A: Fedimint payments surface as LN payments already. You can transact directly ecash but with a LN gateway provider, all payments are LN by default.

User A wants to revive eCash and hits a gateway of Mint A and receives an invoice to provide to User B of Mint B. Mint B’s gateway takes the invoice and does all the normal LN stuff. When the transaction settles, Mint B issues the eCash to User B.

Behind the scenes, both Mint gateways have contracts to swap eCash for LN sats or vice versa, depending on directionality.

In short, if both federations have LN gateways, it is possible to send between them. Just not using the offline ecash system. HOWEVER, if the ecash is sent offline from A to B. B could go to Mint A and claim the ecash then route out to their own Mint or LN wallet.