So I went through the ASME Y14.5-2009 standard and now we are able to fully support it. Each dimension has a new property "StandardAndStyle" which allows the user to select the dimension appearance. Four values are supported in my draft:
1) ISO Oriented = straight line with the text tilted according to ISO 129-1
2) ISO Levelled = a bent line, with horizontal text above the reference line, as specified by ISO 129-1
3) ASME Regular = horizontal text, bent line with small reference line to text vertical center as specifies ASME Y14.5
4) ASME Inlined = straight line with level text on end, or inserted in the middle of line, though not crossed as ASME Y14.5 requires
Here are displayed all (typical) options:
Upper left corner - ISO Levelled
Upper right corner - ASME Regular
Bottom left corner - ISO Oriented
Bottom right corner - ASME Inlined
The dimensoin look is also affected by the additional "Extend To Center" property.
As @Jee-Bee suggested, the standard&style of a dimension is saved in the document, though it can be whenever changed. To improve the user experience, the default value for a newly created dimension can be specified via "Dimensioning Standard and Style" prefference added to page 2 of Tech Draw user settings.
I hope you will like these changes, I've created the pull request: https://github.com/FreeCAD/FreeCAD/pull/2443
Should there be any problems or modification suggestions, please do not hesitate and share them!