With the proviso that I am no longer involved in running WWOOF, so you'd need to check with the current PTB, you certainly didn't have to be registered organic, no. You absolutely did have to be right-on, environmentally attuned and sensitive, etc. But a high proportion of WWOOF hosts are too small for it to be practical to be registered organic. So long as whatever you are, or aren't, is properly and openly written up in your entry in the guide, then it would be up to WWOOFers to decide whether they do or don't wish to WWOOF at your premises.
In the first instance, as a prospective host, you would contact WWOOF UK and discuss with them - by email perhaps, at first - whether you would be likely to be acceptable as a host. If you use a lot of Roundup, it's probably not worth opening a conversation with them, but if you lean towards organic / sustainable / permaculture type principles, then I'd say it was certainly worth discussing with them.