The limb offset commands actually return the offset that you introduce yourself. For example, if you use dbOffsetLimb to move a limb 5 units upwards on the Y axis, then next time dbLimbOffsetY will return 5. But that's not the value you want.
You can try to calculate the difference between the world coordinates of the limb and of the object, e.g.
dbLimbPositionX(1, 21) - dbObjectPositionX(1)
should give the X offset of limb 21 in object 1, as long as the object has not been rotated. I haven't tested it much, I don't know how accurate this is, and I don't know either if there is a better way to get the data.
Just curious, what would you like to use the offsets for?