Local variation is indeed a hint since there are varieties that have more of an [ɑ] than an [ɛ] in <mian>, <lian> and so forth. Secondly, one can indeed analyze at least all the clearly compound finals (as opposed to the five simplex candidates, in Pinyin a, e, yi, yu, wu) as having either a high (as in, raised tongue) or a low (lowered tongue) nuclear vowel. Let's symbolize the former as ɵ and the latter as ᴀ, then Pinyin yin, yan, ying, yang can be analyzed as /iɵn/, /iᴀn/, /iɵŋ/, /iᴀŋ/. PY yong, BTW, comes out as a slightly surprising /üɵŋ/ (with /üᴀŋ/ missing), yue as /üɵ/ (with /üᴀ/ missing), yun and yuan as /üɵn/ and /üᴀn/. One hint that /üɵŋ/ for yong might be a good solution is the observation that Zhuyinfuhao (aka Bopomofo) writes this syllable as ㄩㄥ, which is analyzed (within this orthographic system) as ㄩㄜㆭ, so roughly PY üeng.