Not sure what you mean about "symbolism"

Plumbego capensis is often called leadwort. Lead's chemical symbol is "PB"....hence Plum-Bego

That's the only mildly interesting thing i know about them apart from them being a stunning conservatory plant.

I agree with Andy. Words beginning plumb are from the Latin word plumbum for lead ( PB in chemistry ) and plumbago plants can have a lead -like or graphite colouring sometimes.

A.W.Smith says the name was possibly the result of the plant being used to treat lead poisoning.

Pliny the Elder, however, says that the chewed leaves could be used to 'lick' the eye in order to treat an eye condition called 'lead'.

Gerard, however, thinks that what Pliny meant by Plumbago was Dentillaria (now called Knoxia).

There doesn't seem to be any great folklore attached to it and my Victorian Flower Oracle doesn't include it.