Shane, I don't have access to a B31.3 so cannot help directly but I did work with it for many years and it's my recollection that there are definitions, but not as specific as what you seek.  It seems to me that its the definition deals with defining the toxicity of leakage, even in small amounts, which could lead to fatality... or something along those lines.  Sorry, wish I could be more specific but I'm obviously speaking strictly from memory and a memory thats had a few burnt cells at that! ;-)

I beleive engineering dictates the type of service for B31.3.  In any case, I would definitely have someone tell ME in writing what category it is, as opposed to taking a shot at what I think it is. 

There are two groups that have to make this decision. paragraphs 300(b)1, 300(d)4-5, and 300(e) requires the owners decision, the remainder are the designers responsibility para 300(b)(2).
You can go to appendix M fig M300 for a chart guide to classifying the system. This is out of B31.3 2004

In answer to your question I believe your going to end up fluid class M given all the hoops you have to get through to be otherwise.