Thermal vias in thermal pad of IC?

You can place thermal pad under via knowing that solder paste are not placed on the whole thermal pad surface but is spread in 4 (in this case) the room between each solder paste "island" is used to place via's (this avoid the soldering penetration mentionned by Harald Kapp)

check the PCB layout of the TI evaluation board here you will see that:

http://www.ti.com/lit/ug/snvu179/snvu179.pdf

 

Yes that's it the solder paste island are on the 4 corner for sure.
it' s a common (and needed) practice to have such complex solder mask.

see the figure 13 here under

http://blogs.mentor.com/tom-hausher...ection-starts-in-the-cad-library-part-15-qfn/

Olivier

 

I don't see any reason not to use thermal vias with that device; it seems like a good candidate.

I did a quick web search and found this informative article on thermal vias: http://www.edn.com/electronics-blogs/the-workbench/4421218/PCB-layout-tips-for-thermal-vias