What is the pejorative name given to men who dream up optimistic but poor ideas, such as the speaker in "A Modest Proposal" or the members of the Academy in Lagado?
It's likely that the term you refer to is either "pipe dream" or "pipe dreamer."
The term--which describes an unrealistic, hopeful, but unachievable goal--seems to have originated in England, possibly as a reference to the kinds of dreams caused by the use of opiates. In the late 18th and throughout the 19th centuries in England, one of the unfortunate trade goods coming from China was opium, and derivatives of opium were used and abused by many people, including writers like Samuel Taylor Coleridge, the great Romantic poet.
In Swift's time, the term "projector" had very pejorative connotations because there were so many people who came up with well-intentioned but totally unrealistic ways to solve desperate problems. As your question notes, Swift uses the projector persona as a way of satirizing both the business of projectors in England during the 18thC and the counter-productive ways in which the Irish governed themselves (with much help from England).