This would be great, and I'd be really happy to see it.
One (definitely not insurmountable) problem that would exist in such a federated and open system is credential authentication:
Currently, Apple signs your email address and phone number (hash) so that you can't impersonate somebody's trusted contacts and send unwanted material to them without their consent, which has been a problem for Apple in the past. That's supposedly also why they have removed the "allow all AirDrop senders" option in favor of one that times out after 10 minutes.
There would either have to be a federated alternative to that, or the open source system would have to drop sender authentication; then you could only receive AirDrops while your device is in "allow all senders" mode.
The reason there's anything in the airdrop protocol that can be converted to a person is to allow your device to say who is sending it if you know their identity already, and/or to filter the messages if you don't.
The whole point of this activity was that people did not care, nor want to care, about who was sending payloads. In such an environment the solution is no identity at all, not federation of identity.
If you do try to do this simply because of "federation", all china does it use the same federation system to get the user information (because the whole point here is china was monitoring local bluetooth info, so some nebulous application of federation dust doesn't magically resolve anything).
The problem here is that people were using a system is not anonymous by design (there is a deterministic relationship between the underlying account and the hash by published design), and that relationship is necessary for basic functionality.
A hindsight being 50/50 step could have been to use a password hashing function, but airdrop has existed long enough at this point for me to assume that the iterative systems would have relatively low iteration counts, and mobile hardware probably can't afford the resources to make every airdrop also perform memory bounding steps.
I'm not saying that federation solves the anonymity problem, I'm just saying that the current implementation includes Apple as a trust anchor for email address and phone number verification and issuance of corresponding certificates. My point is that in order to enable an open cross-platform solution, there would have to be some alternative mechanism to that.
What they could add is a sender-side option that makes sending completely anonymously. This would be possible without any change on the receiver side, but would require recipients to enable "allow all senders" mode.
Fundamentally this verification is based on your contact list, which is formed from people you already know and have added to your contacts, so there's not really any need for a centralized trust. Presumably you trust the e-mail address of the contact you added, and the federation protocol could easily define how the authoritative hash/key for each user would be shared based on their e-mail.
In most cases this could also be resolved at first contact in meatspace, directly between the devices when establishing contact via the typical ways users share contact information - QR code or some form of short range networking, or even with an SMS challenge.
Really? It doesn't seem to me like it gets any more trivial than 'hit DNS for the domain to find federation server, send a GET request' to me. You could even do the whole thing with DNS, though that has privacy implications. What am I missing?
One (definitely not insurmountable) problem that would exist in such a federated and open system is credential authentication:
Currently, Apple signs your email address and phone number (hash) so that you can't impersonate somebody's trusted contacts and send unwanted material to them without their consent, which has been a problem for Apple in the past. That's supposedly also why they have removed the "allow all AirDrop senders" option in favor of one that times out after 10 minutes.
There would either have to be a federated alternative to that, or the open source system would have to drop sender authentication; then you could only receive AirDrops while your device is in "allow all senders" mode.