I don't mean to nitpick but if the topic is unusual forms of synaesthesia maybe we aren't actually talking about synaesthesia anymore.
I once gave a presentation on synesthesia in a psychology of perception class, and yes, it doesn't take long looking into the topic before one is smacked in the face with this problem. If I recall correctly, dozens of different types have been named, but it's mostly color-grapheme synesthesia (CGS) that's been studied a lot. So far as I can tell CGS is the most studied because it expresses itself in ways that lend themselves to cheap experiments rather than because it's the only actual type. For example, if a CG synesthete sees red auras around the number 4, the CG synesthete will be able to perceive a triangle shape consisting of number 4's within a random number field:
125
4779
13
47
499
7653899
... much faster than a control.
For the longest time synesthesia was (and to a degree still is) thought about as a merely metaphoric or poetical way of describing perceptual experiences. Honestly, even though I believe synesthesia is quite a real phenomena, I don't think this dismissive interpretation is really that far off, and this is why: the best neurobiological signifier of synesthesia is itself extraordinarily "general." By general I mean that it's been found that CG synesthetes have greater white matter densities (a measure of the degree of interconnectivity) in the inferior temporal cortex. The more extreme the subjectively reported synesthesia, the greater the connectivity measured (by degree I mean whether a synesthete actually sees color auras projected out in the world around graphemes [like a red number 4] or they merely experience a perceptual impression, or association with, color auras [4 "seems to be" red]). However, to talk about synesthesia as the degree of interconnectivity neurobiolgically, and therefore presumably the degree of interconnectivity of perceptual associations, is to talk about nearly everything in consciousness -- because most all experience can be conceived of as "associations."
Obviously we're not talking about "everything" in consciousness, but it's very easy to lose our conceptual boundaries for a subject when speaking in terms of mere associations. It's also been noted that a disproportional number of poets and artists, those who work in metaphors and have powerful analogical minds, are synesthetes. There's a lot of overlap between analogical thought and synesthetic perceptions, and I'm not sure the concepts we have of each can be disentangled without tearing out essential chunks of each other. In line with this generalized conception, some 90 percent of people, when given a choice between the names "Booba" and "Kiki" to assign to each of the below figures names the left figure "Kiki". It seems pretty obvious why: "Kiki" has hard angular consonant sounds and "Booba" has soft rounded vowel and consonant sounds that map onto the visual properties of the shapes. The answer as to WHY it's obvious suggests there is some degree of synesthesia in the entire population, an interpretation consistent with the idea that what underlies synesthesia is a very generalized interconnectivity in the brain (there are also auditory technologies for the blind that that scan objects and convert them into sound forms, which, given enough training, the blind are able to visualize, such that the sound form of a wood saw, for example, can be consistently identified as a wood saw by blind users of the technology [search for "sensory substitution" technology for more info]).
At this point our understanding of synesthesia is not well delineated. So, in my view at least, the best we can say at this point is that the different forms of synesthesia arise from the neurobiological developmental contex in which synaptic pruning occurs. If synaptic pruning is low during a period when a child is learning about colors and numbers you might get a color-grapheme synesthete (I see "4" as red). If it's low when they are learning about numbers and shapes you might get a synesthete who sees numbers as shapes (I see "8" as an octagon). It's telling that most synesthetes outgrow their synesthesia, which is a predictable outcome considering synaptic arborage is much denser in children. As we grow and the arborage is pruned through learning our perceptions and ways of thinking are further crystalized and defined.
It's also interesting that there is a correlation between synesthesia and the degree of nerve tissue in the body. For example, I have a spot on the back of my head where, if I scratch it, I feel an electrical twinge in the middle right side of my back. This kind of nerve tissue interconnectivity is pretty common. There was a
thread in Second Opinion months ago by Seattle_Stranger about a poorly understood phenomenal called "ASMR," where certain sounds and sights can actually trigger a sensory response in 'affected' people that they find relaxing and euphoric. I experience this to a small degree. If anyone else experiences this we had a discussion about it in that thread (in which I draw parallels to synesthesia that you may find interesting).
As far as psychedelics are concerned I believe they mimic synesthesia by blurring associational boundaries between sensations, concepts, and perceptions (as far as these are even different phenomena). In published material on synesthesia authors have noted that only some psychedelic users report synesthetic experiences while tripping, which may indicate that a certain level of innate synesthesia may be prerequisite for experiencing psychedelic synesthesia.