You cannot use such characters in custom document properties as you have discovered, however Charles' suggestion is a good one and mapped controls are easily inserted and then copied and pasted to all the locations required. Using the addin mentioned by Charles
http://www.gmayor.com/insert_content_control_addin.htm you can insert a mapped control and then enter the value into it. You can then lock the control against editing (also available from the add-in of the control's properties) and the value you have entered can be reproduced anywhere in the document by simply copying and pasting and the lock will prevent the value from being accidentally changed (as a change in one mapped control will be reproduced in all the copies).
If you want to change the value, unlock the control from the add-in (which will unlock all the mapped copies also) make the change then lock again. Thus you can use an unlocked control in the document template and lock it as required (or not) when the value has been applied.