I'm not sure if that will work with gtable from CRAN
require(ggplot2)
require(gridBase)
require(gtable)
p <- qplot(1,1)
g <- ggplotGrob(p)
g <- gtable_add_rows(g, unit(2,"in"), nrow(g))
g <- gtable_add_grob(g, rectGrob(),
t = 7, l=4, b=7, r=4)
grid.newpage()
grid.draw(g)
#grid.force()
#grid.ls(grobs=F, viewports=T)
seekViewport("layout.7-4-7-4")
par(plt=gridPLT(), new=TRUE)
plot(rtree(10), "c", FALSE, direction = "u")
upViewport()