Assuming your custom document property is named 'MyValue' and is numeric, you could use four DOCPROPERTY fields coded as:
{ DOCPROPERTY MyValue \* cardtext \* upper}
{ DOCPROPERTY MyValue \* cardtext \* caps}
{ DOCPROPERTY MyValue \* cardtext \* lower}
{ DOCPROPERTY MyValue \* cardtext \* lower}

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]