If there is someway you can attach a small pc of styrofoam under the shirt you could use bamboo skewers (sp?) to stick in the shirt and they should stay, attach a small wooden ball at the top for the tip of the pin.
sorry, not sure how comfortable it would be.
Not sure how safe that would be, either. Trying to figure out something that has the appearance of pins, but won't accidentally impale you if someone bumps into it. Maybe something like stiff plastic tubing.
I thought about this a lot last year for a rag doll costume that never came to be, (so I haven't actually tested it, keep that in mind) and here's my theory:
You use metal or wooden knitting needles for the pins, and you'll need eyelets to set them (so get a gauge of needle that is the right size to fit tightly into the eyelets). Then you'll need a close-fitting vest of some sort under the shirt or a kind of harness. If you or a friend has and is comfortable with the appropriate tools, you can cut them in half or at least sand the ends down enough to avoid poking yourself with them*. They'll be longer if you just sand the end, it depends on the effect you want. Put an eyelet about where you would want the pin to stick out of your body, then put the flat end of your needle/needle half in the eyelet and secure it with some hardcore adhesive of your choice. If it still moves around more than you want (harness/vest not tight enough) you can either try adjusting the harness or stick the spot to your body with prosthetic adhesive stuff, since most of the support you'd need is already there to hold the needle in place... though I would recommend some sort of padding if you want to avoid actually being poked.
If you cut the needle in half, you can have the blunt half on one side of your body and the pointed half sticking out on the other side, making it look like you've been run through. It'll just look like flat-head pins this way but you could add whatever bauble you want to the end of the needle for decoration.
*Alternatively, if you can't cut or sand anything, you could just feed the needle through the eyelet with the pointed end sticking out and the flat end on the inside of the vest, which could make it easier to glue and attach to skin. Then, you could just cover up any pointy ends that you don't want to be pointy with whatever.
I did a voodoo doll a few years back and the way I did the pins was by using wire coat hangers and small foam balls glued and painted on the ends. I wore burlap and just threaded the pin through the fabric. For the ones that looked stuck in, I cut them short, cut a small block of pink foam and duct taped it the the inside of my shirt. Then push pin into that. For safety sake, I bent the back of the pin into an L shape and duct tape that to the back of the foam block.
Hope that helps.
Also, for added affect, I made a small voodoo doll that looked just like me and used it as a prop.
I think DeadMeat's idea is great, and to add to it I would suggest bending the coathangers in a squared off U shape with both tips poking thru, so they will be better supported and impossible to pierce you... and if you bend a 90 in the center part of the U it would help keep the U from flopping over...