Well if can confirm changing the values in the build.prop to this:
Does not gain compatibility to the nvida shield games on the android market, and does not cause any issues on the mojo.
Only thing I can think of now it to aquire a copy of half life 2 apk file and see if when its side loaded its tricked into thinking its loading onto a shield. But in an ideal world I would like to purchase it from the store to support the devs
what determines market compatibility is the keys;
ro.build.description=d2mtr-user 4.1.2 JZO54K R530MVQAMF2 release-keys
if you were to swap those with the lines you have the market thinking you have metropcs CDMA SGS3 on 4.1.2. i would suggest trying to grab a rom from the shield and swapping the values you have for those ones .
ro.build.description=thor-user 4.2.1 JOP40D 8857_206.8556 release-keys
there ya go, the shield from 4.2.1, it wa steh only rom i could find for em but at the very least its worth a try make sure you backup your build.prop before editing em.