Once you get above 1.003, your list of plants gets very short. Up to that level, pretty much anything tolerant of hard, alkaline water is worth trying. At 1.005 you want to try those species -- such as Java ferns, Crypocoryne ciliata, hornwort, and Crinum calamistratum -- that actually come from brackish waters in the wild. Above 1.005, there really isn't anything reliable (other than mangroves).
In the Aqualog brackish book, mention is made of Samolus valerandi being tolerant of a salt concentration of up to 30 ppt (~ 4/5ths normal seawater) but this is otherwise a difficult plant to raise in aquaria.
To be honest, I'd stick with plastic plants.
Cheers,
Neale