Extending urdf with semantics has been discussed here as well:
http://ros.org/wiki/srdf/review

I am not sure whether such contact points could be rethought as end
effectors in the srdf format.

> Hello,
> here is another proposal to enhance humanoids robots support in ROS.
>
> When dealing with legged robots, one very important information is the
> list of valid contact zones, i.e. the zones which can safely support the
> robot.
> Information about soles are also required to generate walking
> trajectories.
> An example of this would be
> http://hal.inria.fr/docs/00/46/41/25/PDF/paper.pdf.
>
> I suggest adding to the <link> element of URDF an optional node
> called "contact" which structure is very close to the geometry and
> collision elements.
>
> The attached document specify how this could be implemented.
>
> If you feel like this should be a REP, I can adapt this draft document
> to the REP format.
>
> If this new node is accepted, I can provide a set of patches for the
> urdf, urdf_interface
> and urdf_parser packages.
>
> Best,
> --
> Thomas Moulard
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