It's not the same thing. the si serves a very specific purpose. It's not a pain thing, or a modification thing, but a "relief from feeling the assault again" thing. body mods in that area would not work... also, i have never associated body mods with self-harm. Once I got a tattoo because I was trying not to self-harm, but it was not the same outcome (emotionally).
I think the body mods would be more effective for the sexual pleasure piece though, and have thought off that. It's just too difficult to have someone else anywhere near that area (other than my wife, and even that is hard a lot of the time)