There's a song by Steve Vai called "Little Green Men" in which he mimics two voices talking through bad radio. I like doing a simmilar (no trem) thing with wah, distortion and the pick. I call the wound strings "male voice", and others "female voice", and then I play with them. You can use other things instead of the pick too. Try bouncing a metal pen on a high e string.
When I want to go higher than the fretboard range I slide the pick upwards on a high string and stop when I reach the desired note, continuing to scratch it in one place to produce the tone.
Try scratching with your nails as well.