Smelvadj:
Maybe I'm missing something, but I played around a bit with this and had no trouble displaying out to eight decimals. I saw two ways:
On the shapesheet, in the Shape Data section, in the Format cell, I entered the value ="0.0000000" (with equals sign and quotes), and set the Value cell to 0.00000010. When I display the shape data for the shape via right click, it shows that correct value. I can also change it, and it does so correctly.
Likewise, in any other shapesheet field, I used the FORMAT function to do the same thing, such as =FORMAT(User.TestField,"0.00000000") such that when I assign 0.000001 to User.TestField, the FORMATted cell contains the correct decimal value. It also works in the Text Fields section when the tiny decimal is displayed as the shape's text. Granted, sometimes Visio will round it to the nearest wrong value, but the FORMAT pictures override that.
More info about the FORMAT function pictures can be found
here.
Good luck!
- Ken