I think the most important question to address is in the last paragraph: "If palatal vs. alveolo-palatal is not a contrast in any language, does that mean there's no point in trying to find a feature that distinguishes the two?" More generally, what is the point of features? Because there is no agreement in the answer to that question, then you will get very many different answers the the issue of interest to you. The second most important question, which also doesn't have a particularly good answer, is what does the distinction ɕ, ç even refer to? IPA does officially recognise those two symbols. But there is no official or defining set of recordings adopted by the IPA as setting the standards for these sounds. Thus it falls to the late Peter Ladefoged, who was arguably in the best position to speak to what an IPA symbol "meant".
On p. 150 of The sounds of the world's languages, ɕ is discussed in connection with some speakers of Standard Chinese: the alveolo-palatal is referred to him as "palatalized post-alveolar" (which is parenthetically equated with alveolo-palatal). In other words, it is equivalent to ʃʲ. Then you just have to settle on a theory of how to say (in features) "palatalized post-alveolar". There was a standard set of features promulgated in Sound Pattern of English, where ʃʲ = ɕ is [+coronal,-anterior,-back,+high] and [+high] is the essential feature distinguishing ʃ from ʃʲ. The palatal fricative ç, on the other hand, is [-coronal,-anterior,-back,+high]. Subsequently, many competing theories of features entered the marked, so really there is little hope for answering the question if you are looking for a consensus.
In substance-free theories of features, there is in principle no way to answer the question except by reference to how the sounds are distinguished in a language. As far as we know, there is never any contrast, so little hope of answering the question. Still, there might be tiny bits of hope out there, based on phonological classes. Sanskrit has palatal consonants conventionally transcribed as <c j ś> where the latter (श) is IPA (also Whitney's grammar) ç. There are no "alveolo-palatal stops" in IPA, so we know that <c, j> "have to" be treated as true palatals, i.e. [-coronal,-back,+hi]. But phonologically speaking, <ś, श> has the same place of articulation as <c,j> with respect to consonant sandhi rules. Thus we can identify the features of this particular instance of ç based on behavior. Often, there is no such argument, for instance (dialects of) Norwegian has <kj> which is pronounced [ç,ɕ], but for which there is no behavioral evidence telling you what is acts like. The substance-free analysis in this case is that there is no feature in Norwegian which say anything more specific (assuming a dialect with a 3-way contrast [ʃ ʂ ç] as opposed to the 1-fricative varieties).