@jlawler pretty much answered this already in his comment, but I thought I'd post an official answer.
In theory, yes--it's sensible to apply the concept of homorganic-ness (homorganism??) to things other than consonants. As you mention, one could consider the offglide in the diphthong [ɑɪ] to be homorganic with the vowel [ɪ], or the glide [w] to be homorganic with the vowel [u].
In my experience, however, the term is rarely used in this way. One possible reason is that vowels and approximants are more difficult to characterize in terms of "place of articulation". What is the "place of articulation" of [a], for example? or of [ɹ]? And as for the diphthong [ɑɪ], some people transcribe it as [aj]. Its pronunciation can be more like [ɑɪ] or more like [ai]/[aj] (i.e., with a more extreme high-front offglide) depending on the context or how carefully it's pronounced, even for the same speaker. Does that mean that the former is homorganic with [ɪ] and the latter is homorganic with [i] and [j]? What would such a distinction be useful for?
Which brings up a more general point--a label like homorganic is meaningful in linguistics only inasmuch as it is useful for delineating certain properties of speech/language units or relationships between/among speech/language units. Phonologists started noticing that consonants often form natural classes along certain dimensions--one of them being "place of articulation". That is, they noticed that consonants whose oral constrictions are made in the same or similar place in the mouth tend to group together in terms of the phonological processes they undergo/trigger. And they also noticed that certain phonological processes are sensitive to sequences of consonants that have the same place of articulation, hence the usefulness of the term homorganic. (In phonetics it is also sometimes useful to identify homorganic relationships among consonants, as doing so may bring certain phonetic patterns to light.) When it comes to vowels, phonologists found it more useful to categorize them along several "place-related" dimensions, like height, backness, etc. As @jlawler pointed out, if two vowels are articulated at the exact same place, they are trivially the same vowel and trivially "homorganic"; but then in that context the label is not very useful. The exception I can think of is a nasalized/non-nasalized pair. I'd wager, however, that it's unlikely that you'll find a process involving sequences of nasalized/non-nasalized vowel pairs in a language that makes such a distinction phonemically!