How about the big elephant in Bitcoin's room. Is there a way a SpreadCoin service node could incentivise somebody to run a full bitcoin node?
Interesting question.
Running a full bitcoin node will always be a purely voluntary act.
There is also a feedback effect in work: people who are longterm invested in BTC are also
automatically the people who will run a full bitcoin node (or keep on mining) indefinitely,
because for them this is like some kind of insurance.
They understand that without their help the perceived value of the BTC they hold vanishes...
So in their mind, I don't think that they need any additional incentives to do something that is healthy for the network.
So your question I guess is more about... how can we incentivise someone to run a full bitcoin node who doesn't own any BTC at all?
....
Now, if we as a network of a few thousand servicenodes could have a bitcoin node installed on each of our service nodes, than this would serve bitcoin well.
Maybe if this decentralized searchengine is also a giant decentralized blockexplorer? Then we could help all cryptocoins (we take in) and they would be incentivised to help us... by buying SPR I guess...