nlp - opinion mining vs sentiment analysis
Can someone point me in the direction of more sentiment analysis
  oriented resources?


There are tonnes of exceptionally good papers on sentiment analysis for your reference. Especially the ones on social network sites like Twitter.

However, I think the book by Manning and his course are a nice start as references to NLP and opinion mining.


  Can nlp, or something else, be used to derive unstated affect?


Yes, it can be. This would be my approach:

(Consider I already have a hand-classified training data) Using the bag of words approach, the most used words and phrases for a mood (stressful, jovial, etc) can be captured, and the subsequent training data can be ranked accordingly as (60% stressful, 40% jovial).

Additionally, you can also define thresholds for getting a single mood. Like:

if mood[score] &gt; .5:
    current_mood = mood