The idea of the "Guardian" is good, but I am not 100% convinced to make this to a requirement for the term deposit:

1.
Who says that only running the wallet will be accepted as "work" from the US goverment?

2.
Who says that it has to be 60% of the deposit time? Perhaps 10% would be enough for the US goverment or perhaps 80% is not enough for the US goverment.

3.
I think the term deposit is one of the main features of ERC and very attractive for new users. But when the users have to open the wallet for 14-15 hours a day, this seems not to be very attractive in my opinion. Many "normal" user have not running their PC/Laptop so many hours a day and now they should keep it running the whole day only for the ERC wallet?

4.
Also it seems to be a little bit risky. For example I make a deposit for one year. And after one year I only have 58% running time because of holiday, pc was broken, forget to restart after crash, wallet hanged for some days and I don't recognised it, ... Then I don't get any reward for my deposit even I had my wallet running for over 5000 hours and my coins were locked for 1 year.

I would prefer a solution where the wallet has not to run all the time. But I also have no good idea at the moment when we don't know 100% what is accepted as "work".