Tricky question.
Firstly, if an up-vote (thumbs up) option is being considered, it would be remiss not to consider a down-vote (thumbs down) option.
The risky part of such behavior is that contentious discussions (such as the one of the EU referendum) may become more visibly divisive by listing how many votes have been cast for and against.
An alternative would be a "thank" option, where a post can be "thanked" for useful information.
This link provides some technical background for PHPBB forums.
Overall, I am leaning against the idea on the grounds that it will only cause bad blood and factionalism on these forums. But I am open to being persuaded otherwise.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.