Has anyone ever gone from Summoner to Dragon Disciple?
It seems to me that it would not work very well. When the Dragon Disciple gains a level in his spellcasting class, he gains spells known, spells per day and caster level, but he would not improve the Eidolon at all.
I'd advise you to check the Guides to the guides in this sub-forum for Oterisk's guide to the DD, he describes the pros and cons for a DD based on a summoner.
I could see a Dragon Disciple 4-level dip for a non-Synthesist summoner if you want to have more melee ability at the cost of relatively few spells. Extra strength, some fancy abilities, and not TOO much progression loss on the eidolon.
It isn't optimal, true, but it definitely seems viable.