...Should he default on payments then a lawsuit could be filed...
It is very hypothetical if you think any judge is going to hear a case about losing a virtual currency. I consider anything denominated in bitcoin as a game and I treat it as such. This game gets bigger and better everyday and we are all participants that contribute to the design and the gameplay.
There is nothing hypothetical about it and there is a long tradition in the common law of countries such as the United States, UK, Australia and many other jurisdictions. For a contract to be formed there must be offer, acceptance and consideration. Consideration can be anything of value promised to another when making a contract and can take the form of money, physical objects, services, promised actions, abstinence from a future action, and much more.
In this case, the promise of a coupon payment equal to the processing power relative to the difficulty is of value and pretty easily determinable for damages purposes.
On another note, yesterday I had lunch with two attorney friends and Bitcoin was the topic of conversation. Both mainly focus on asset protection and estate planning but one has extensive experience securitizing all types of assets. And we did have some tangent discussions on the securitization of mining bonds.
So as the Bitcoin economy continues to 'grow-up' I think we will only see an increase of professionalism from the participants and ventures.