Hi @remlaps, this is a very interesting idea...

I think if I were the one making the decision, I'd allow voting for any content in order to attract users, but impose a cap on the voting percentage for non-exclusive content. For example:

Max vote = 100% for exclusive Steem content, Max = 50% for content that's also found elsewhere.

And in the comment, the curator would note that the post could have earned a higher upvote if the post had been exclusive to Steem.

If you would like to try that approach, we would be in full support, and would be interested to see how it works for you.

Give it a go and keep us posted.

Thank you

The Steemit Team