IncX, what is your string height? I have seen other guys using the fretboard as a ramp, but whenever I try to do it I find that my strings are too low to pluck them from below with my thumb over the fretboard. And I don't have a particularly massive thumb. I am currently doing it over the neck pickup, but I feel that the string is probably a bit too tight there, cause it is closer to the bridge compared to the space around the 24th fret and I would really like to have a bit more "slack" in the string when I do double thumb.
I am seriously considering adding a Norm Stockton - style ramp to my $$.