[ashwken]

Just a thought, what if you used the underscore character(s) to preceed each item in your check list:

___ - Thing to do

___ - Next thing to do

Granted, it's not a checkbox, but gives you the same functionality and the "list" can be created in the Detail Pane of a single Item.

(Added)
The only other possibility would be to use the Windows CharMap program, but the "white box" available in the Ariel font is not very big, would probably need to open the WingDings font.

CharMap does allow you to copy the desire character to the Clipboard, and UR will accept it in the rtf editor from a CTRL-V, maybe in conjunction with a Template might give you what you want.