hm... oddly enough, i don't think it's "completely" possible either.
Try this:
1) create your text. Just type the number 1 for a test. Make it very large -- 500pt.
2) Using Layer Styles, Stroke it by 2px. Apply.
3) Right-click the type layer and RAsterize it, then right-click again and choose "Create Layer". You'll get 2 layers -- 1 your original type layer, and 2 your expanded copy that creates the stroke effect. Click on that layer.
4) FILTER>Other>Minimum. Set it to 4px.
Now check out what's happened here. All of the corners except the top-front get 'squared' somewhat decently. Just not 'completely'. (see example pic)
I tried a number of methods and they all produce rounded results; except the method i just described.
I find this odd considering PS is pixel based, and pixels are square.
My conclusion then... is that it has everything to do with PS's lack of anti-aliasing control. It's preventing the corners from being rendered 'pure'.