Though most OSes support Unicode adequately, the software of many online forums and boards doesn't. That's why I find using SAMPA preferable to using ad-hoc English orthographese like enya for eñe (or tilde).

Ok, it sounds like you are talking about the nasal quality of much American English. I've also heard this described as a Midwestern feature. By definition it's not represented in standard written English. I haven't seen it in attempts to write down dialect either. I think Americans are not conscious of it as a phonemic feature, as opposed to a general tendency.

Americans learning French have a terrible time with the distinction between nasalized and unnasalized vowels. First, they're not conscious of it as a phonemic feature, and second, they're often told something like "breathe out a little through your nose while pronouncing the vowel" and can't figure out why they are unable to add more nasalization to their already (unconsciously) nasal American vowels.

Are you asking how nasalization is represented in IPA? en.wikipedia.org/wiki/Nasalization