Hi Gareth,
I've been asked to pop by this discussion. You've asked a great question, Others have contributed sound advice here. Your client is encountering some of the limitations of Google Places. Yes, you are restricted to 5 main categories, and in a sense, this system favors businesses with a narrower focus over those with a much broader one (like your client). The method that you have undertaken of focusing the Place Page on the business' 5 most important categories is correct, and then using organic SEO to pursue what else you can. But, because of the heavy slant towards local results in the SERPs, those things for which your client is not getting a mapped result will be at the bottom of the page or even further down than that. There isn't really a way to get around this that I know of, and I don't see Google changing this layout for some time to come.
Regarding the additional details field, it's my own practice with my clients to keep this pretty brief. I do believe that a stuffed additional details section could send a danger signal to Google, but what the threshold is, nobody knows. In any case, I do not believe these details carry a huge deal of weight as compared to, say, the contents of the website, particularly in competitive verticals. Now, if the business was the only one of its kind in a rural area about which Google had a paucity of information - then, yes, I wouldn't be surprised to see those additional details gaining a little traction. But in a competitive vertical in a suburban or metropolitan area, I would not expect anything you put in there to enable the client to outrank a competitor who has chosen one of the phrases as one if his top 5 specialties.
This is one of those areas in which it is critical to correctly the set the client's expectations. If they offer 25 different services, they should not expect to be #1 for all of them unless they are the only game in town. Google's results are more democratic than this, so the client is forced by the system to zone in on what their most popular services are with their Place Page, and do the best they can with organic for the rest. Don't forget, traffic comes from video marketing, social media and other forms of marketing, too! If the client wants to highlight other services, a creative approach to finding ways to promote them beyond Places deserves some thought.
Hope this helps, and good luck!
Miriam