A datum curve's dimensions can be shown decimal or fractional like any other dimension. So, by equating the length of the datum curve to a parameter, I can effectively show the parameter's value in either decimal or fractional format. Here's a simple example of what I do:
1. Start with a part or assembly.
2. Establish key parameters via inputs, relations, or whatever. Some relations for the example:
- D0=2.5
- D1=5.125
- D2=7.75
and
- AREA_TOP=D1*D2
- AREA_RIGHT=D0*D2
- AREA_FRONT=D0*D1
3. Add datum curves (lines, rectangles, or whatever) with enough dimensions to represent the key parameters.
4. Equate the datum curve lengths to the parameter values. Some more relations:
- D10=AREA_TOP
- D11=AREA_RIGHT
- D12=AREA_FRONT
5. Hide the layer containing the datum curves so they don't make a mess of the drawing.
6. Display the values on your drawing in a note or table.
7. Select the resultant values in the note, right-click, and select Properties to change between decimal & fractional.
7. Regenerate. Done.
A little tedious having to include Steps 3 - 5 in the usual drawing-making routine. But that's why it's a workaround, not a solution. 
Steve