I think this is a bug. (Or a consequence of the fact that “multi-line expressions are not supported”, as stated in the conversation you linked to).
The workaround that Gavin Simpson alluded to is:
#For convenience redefine p as the unlabeled plot
p <- ggplot(mtcars,aes(x=wt,y=mpg))+geom_point()
#Use atop to fake a line break
p + xlab(expression(atop("A long string of text for the purpose", paste("of illustrating my point" [reported]))))
It is possible to use true line breaks with subscripts. In the short example below, which has the same form as your example, the subscript is correctly placed adjacent to the rest of the text but the two lines of text are not centered correctly:
p + xlab(expression(paste("line1 \n line2 a" [b])))
I think that in both cases, the subscript is placed wrong when the upper line of text is longer than the lower line of text. Compare
p + xlab(expression(paste("abc \n abcd" [reported])))
p + xlab(expression(paste("abc \n ab" [reported])))
The subscript always ends up aligned just to the right of the right end of the upper line.
p + xlab(expression(paste("abcdefghijklmnop \n ab" [reported])))
