I may have spoken too soon. Here's how to enter an en, em or hyphen dash.
First enable the Japanese keyboard. (Settings>General>Keyboard>International Keyboards>Japanese[ON])
Then when entering text in English, press the Globe symbol (between the [123] and the [space] keys). This will bring up a slightly different keyboard. If you have other keyboards enabled, cycle through them until the Japanese one shows up. You can tell which one is active by the lettering on the [space] key. On the Japanese keyboard, you'll see a dash key immediately to the right of the [L]
Touching this key will bring up a screen above the keyboard showing keys with three different dash lengths. Simply select and touch whichever one you want and the hyphen, en or em will be entered into your text.
You can then switch back to the English keyboard and continue on.