The Halruaans are decendants of the Netherese. Given that they are also a magocracy ruled by 400+ high-level wizards, and that they employ magic, such as flying skyships, at a level unlike any other society in Faerûn, it is a good bet that some of the Halruaans might know of the secrets of building mythallars.
I am not sure what happened to him during the Spellplague and the Second Sundering, but Raumark, the sorcerer-king who led the Netherese down to the Shining South and found Halruaa, was still alive and kicking (in the hallucinations of Elminster?) in the book Temptation of Elminster. According to 3.5e sourcebook Lost Empires of Faerûn (LEoF), Raumark was the Archwizard of Hlondath, one of the three flying cities that the reincarnated Mystra could save following Karsus's Folly.
LEoF has a section titled "Keepers of the Past" (pg 100-102). It first mentions a number Netherese characters who are still around: Aumvor the Undying, several members of the Grand Cabal, Telamont Tanthul. Then details are provided for a couple of other Netheril survivors who might be willing to share information with others: Ioulaum (as an undead elder brain); Rhaugilath; the ghost sorceress Lady Saharel ("a leading member of the High Mages of Netheril"); and the sarrukh lich Arthindol, who used to be known as the Terraseer in ancient Netheril. All of these archwizards have been around for a long time (in Arthindol's case, for more than 30 thousand years), so it would be reasonable to assume that they must have weathered the weird D&D4e century, as well. Finally, one of the greatest storehouses of Netheril's lost lore, The Library of Raumark, is described to be at Mount Talath in Halruaa.