When identifying the themed relevance of a link Google takes into account the title ,content ,anchor text of the page and the actual link of the text. But seeing as you are dealing with a really niche market it's probably safe to say that you can't find many linking opportunities that cover all these criteria.
If you've got the opportunity of sites linking to you using your keywords in the link text and bearing in mind they have something to offer visitors to your site and a decent PR, then I'd say go for it.
It's my opinion that links do not always have to be within a theme. It just depends why it's there in the first place, is it for Google or for visitors to your site.
Thanks, thats my opinion also. For 1 term in particular we had no SERPs to speak of and this last month we got one link (semi-related site) in particular that has the term in the link text. We got to number 3 in the big G!