If you are prepared to put the work in (you are downplaying the the time and effort potentially involved I feel) it sounds like a good idea. Particularly if there is nothing available in Portuguese.
What sort of information do you envisage disseminating in Portuguese? Opinion or only official Scotland Yard backed appeals, press releases (on which their hands are apparently tied - perhaps in anticipation of legal action in Portugal).
The same board officials who currently have the power to edit posts, delete posts and delete threads would retain control.
At the moment, the Crimewatch programme appears to be an example where OG would like to have such an appeal within Portugal, but whatever is preventing it is preventing it.
The forum has the ability to overcome this obstacle, quite legally. The question is whether it has the desire.
Just to round out, where an event is widely reported in Portuguese media, say the Dec 2013 interviews in Faro, I can see no value in adding another layer here, on the same topic, written in Portuguese. From memory, I wrote a separate article on each of the 10 persons interviewed, in English, on my blog. Since OG has a better understanding of who these people were, why they were interviewed, and what OG got from them (which I don't know), then translating everything I wrote into Portuguese probably would not lead to any new information surfacing.