If I understand correctly, that is exactly what Kaching (now wealthfront) did in the beginning. I've heard of a couple of other startups doing basically the same, but the only one that ever had any traction AFAIK is kaching.
There's a regulatory minefield out there that would have to be cleared, e.g., if I myself am sharing my portfolio, I would need a really good opinion that I'm not on the hook for going "investment advice" by the SEC.
Also, I subscribe to a couple of investment newsletters - I might be in violation on their TOS if I "publish" their advice automatically, even if I keep their analysis to myself. Completely automatic publishing would be unacceptable for me - it needs to be at least screened by ticker / market, and probably at the individual trade level.