The ones that *could* pay non-mining full nodes are miners/pools, by
outsourcing transaction selection using a different PoW.  By doing so
they could buy proof-of-uncensored-selection and proof-of-goodwill for a
small fee.
We would allow full nodes to generate and broadcast a template
block which:
* Does not contain a valid header yet
* Contains the transaction selection
* Contains a  coinbase output with a predetermined part of the block
  reward (say 0.5%) to themselves* Contains a nonce for PoW of a predetermined currently ASIC resistant
  hash function behind a OP_RETURN.
The template with the highest PoW since the last block would be leading.
A miner/pool can then choose to use this instead of their own, adding
the rest of the reward and the SHA nonce themselves. That way they would
set up a competition among full nodes.
This would of course be voluntary but provable, so maybe in a pool's
interest to do this via naming and shaming.
Tomas
bitcrust

On Wed, May 3, 2017, at 23:43, Ben Thompson via bitcoin-dev wrote:
> I feel like this would be pointless as the vast majority of users
> would likely download the blockchain from a node that was not
> enforcing a tip requirement as it would seem like unnecessary cost as
> in protocols such as BitTorrent there is no such tips in sharing files
> and the blockchain distribution is in eccense the same thing. However
> perhaps I am underestimating the generosity of node operators but I
> feel that adding a cost to the blockchain (assuming that all users add
> a tip requirement) would lead to centralisation.> 
> On Wed, 3 May 2017, 22:21 Erik Aronesty via bitcoin-dev, <bitcoin-
> dev@lists.linuxfoundation.org> wrote:>> IDEA:
>> - Full nodes advertise a bitcoin address.   Users that need to
>>   download the block chain from that node can be encouraged to send a
>>   tip to the peers that served them (by % served).   Recommended tip
>>   of 10mbit should be fine.>> 
>> - A full nodes can *require* a tip to download the blockchain.  If
>>   they do, users that don't specify a tip cannot use them.>> 
>> CONS:
>> 
>> For some people, this may represent a barrier to hosting their own
>> full node.   After all, if you have to pay $15 just to get a copy of
>> the blockchain, that just adds to the already expensive prospect of
>> hosting a full node.>> PROS: 
>> 
>> As long as you manage to stay online, you should get your money back
>> and more.   This is the an incentive for quality, long term hosting.>> In the long term, this should cause stable nodes to stick around
>> longer.   It also discourages "installation spam" attacks on the
>> network.>> Fees for other node operations can be considered if this is
>> successful.>>