For the non-Arabic speakers of this thread:
Originally Posted by Wadi Hanifa
"And among them are those that don't lead it to its place [of articulation], but rather produce it without it[s original? place of articulation], mixed with the lax ṭā; they don't regard it as other than that; they are most Egyptians and some from North Africa (the Maghreb)."
Fascinating. These comments point to a possible Egyptian origin for the plosive version of the sound. It seems elsewhere it's always some kind of fricative. Also in all of the Indo-Iranian languages that have borrowed from Arabic it becomes a [z] sound I believe.
Anyhow, it's possible we should decide to split off this thread into a proper discussion of the ظاء and ضاد merger or just about ضاد in particular. To steer this back on topic, it seems there has been a consensus that ḍād was an emphatic fricative in the past which is a more likely candidate for the emphatic version of one of the Proto-Semitic "s"-like sounds which was the point of being brought up in this discussion in the first place. It also quite nicely rationalizes how such as sound which was at one point preserved in Arabic, in other Semitic languages like Hebrew actually merged with ṣ ص. Not that we needed such a rationalization, but it fits with the story of the evolution of Semitic languages. The voicing feature in Arabic I think is what made it more likely to merge with other voiced sounds like ظ or ز or even ط which was voiced according to Sibawayh, rather than ص.