flohmarkt now implements FEP-0837: Federated Marketplace!
They add Proposal as an attachment to Note. Here's an example:
https://kleinanzeigen.bonn.digital/~bonndigital/48a0e2b7-f0d0-421b-9190-cc153...
@silverpill And use a multi-type then, too! :) I am becomming a fan of those more and more.