Self-serving plug: I created valME.io to do exactly this. [1] It uses Bitcoin. [2] YoursNetwork also has a working prototype. [3]
[1] https://valme.io/c/gettingstarted/faq/pkqqs/the-genesis-of-v...
[2] https://valme.io/c/gettingstarted/faq/kqqqs/how-valme-works/