Another mac/note-taking question

Hello Core. As a recent owner of a Macbook Air (which I love, by the way--thanks again for all the advice), I am re-doing my workflow. One snag is that I cannot find a good (for me) note-taking software, despite the proliferation of such on OSX.

I have tried all the usual suspects (Evernote, various Simplenote clients, Curio, Omnigraffer, DayOne, VodooPad, DevonthinkPro, etc. They all have one significant weakness (again, to me): They do not allow me to apply tags or the equivalent to part of a note. Just about every note-taking software allows tags on the note-level, only, not on part of the note. It seems like Growly should be able to do this, but I cannot for the life of me figure out how.

Is there an app out there that would allow this, short of various $500+ qualitative data analysis programs like HyperResearch? Or is there a way to set this up with Multimarkdown or something like that? Tagging part of a note is really my only requirement, but things like an iPad app or cross-platform would be nice as well. Some rudimentary analytics (e.g. how frequently a tag appears in conjunction with another tag) would be super, but I am probably asking for too much. Many thanks for your help.