^^^
I'm looking at this scenario from the perspective that GRRM would expect his educated audience to ponder . . . like the clues he puts out there about the Lady Ashara Dayne not being nailed to the floor at Starfall during all the time which transpired from the Great Tournament at Harrenhall and her alleged suicide (he expects us to think and ponder)
With her predicament (Her husband King Aerys II was unquestionably insane and unfit to save House Targaryen and her older son the Crown Prince Rhaegar Targaryen was slain at the Ruby Ford) she has to be the one to assume the mantle as head of the family for the very survival of her eight year old prince and potentially another prince in utero.
She had nine months to plot & plan. She had a maester and ravens at Dragonstone . . .so who did she write to? She had soldiers and knights and a Navy and a treasury of sorts too. I'm saying in all seriousness that Queen Rhaella, Second of her Name did not spend nine months on Dragonstone just knitting baby blankets . . .she contacted people she knew and took steps to ensure the survival of her House. I'm thinking she sent a raven to the Prince of Dorne because he was the son of her dear old friend who along with Johanna Lannister was her lady-in-waiting for years while she was a Targaryen princess living at court. Thoughts??