[NTG-context] Asking context to switch to a different font for a single character in many places

Hi, I am trying to prepare a moderate sized document in Malayalam using Context. Overall I have been successful. However, there are a few rough edges for which I need help. This is the fifth problem I face. Is it possible to instruct Context to use a font in general, except for a single character? I want to show the space character with open box,␣ U+2423. However, the font doesn't have that character. The open box character shows up in many places within an otherwise Malayalam text. So, switching font is not possible. For the time being, I am using a substitution. However, I would like to know if it is possible. Thanks, ajith