A common practice to evaluate keyword difficulty is to assess how many websites are deliberately targeting this keyword. In other words, when one searches for a keyword or key-phrase in Google he may get results from websites that haven't targeted this keyword (although they may target other keywords). Therefore, in order to find its "true competitors" one should count the pages that use this keyword in their title, URL, or domain name, backlinks anchor text, etc.
Still, some pages may appear in the first page mainly for their domain authority or domain trust (e.g. Wikipedia). Now, what if we could utilize the potential of these websites for our offsite SEO? Indeed, earning a backlink from Wikipedia or a major newspaper may be very hard (especially for new websites), but we may still find domains that "supports" certain keywords although they are not fully optimized for them.
Guest blogging is one of the most efficient ways to obtain backlinks from highly authoritative blogs. Moreover, it provides an opportunity to optimize entire webpage on other's domain. Writing and optimizing a guest post for an "unfulfilled" keyword on other's website would increase its odds to appear on top of the search results since it would combine both -- domain authority and keyword optimization.
Now, I'm not advising guest writers to use "keyword stuffing" or other black-hat SEO techniques. What I'm really suggesting is a reasonable optimization that will benefit both -- guest bloggers and their publishers. Actually if the post will be optimized right, it would outrank posts from the publisher's blog, that are not fully optimized for this keyword. Still, it would be optimized his blog for this specific keyword.
In order to find the unfulfiled blogs one should simply locate the web-pages in the SERP that don't include the keyword in their title or backlinks. A nice shortcut would be to use the operators intitle: inanchor: (i.e. [keyword -intitle:keyword -inanchor:keyword]). However not every blog is opened to guest posts. That's where Google Custom search engines (CSE) can come in to assistance. By setting a Google CSE that lists all the major blogs in his niche that accept guest posts, one may find the most powerful blogs for his desired keyword that are opened to guest bloggers.
A convenient way to collect such blogs would be to use my ready made query or this search form. I would also recommend to assemble as much blogs as one can in in order to find the blogs with the higher potential for a given keyword later. (Although it may seem very tedious for some readers, this work may be worthwhile in the long term since one may reuse this CSE for his future guest posts.)
Last but but not least, some will argue that optimizing your publisher blog for specific keyword may hurt your odds to rank high for the same keyword. As a side note, this dilemma is not unique to guest blogging and it's relevant for every reciprocal exchange, may it be link exchange or business partnership. Also, the answer for this question is depended partly on one's world view. Anyway, if the publisher's website is ranked significantly higher than the guest's websites, I think that optimizing the guest post would not hurt the writer. Even if the guest's website will rank higher for this keyword in the future it would take several months or even years to rank above the fold (i.e. in the top three results). By that time the publisher's site may be hurt by Google update or by a competitor. Moreover, you and your publisher may rank both #1 at the same time due to geotargeting and personalization influences .