I've never looked into the NavBar that deeply. In any case, I don't think that there is anything in build.prop for that. About the only thing in build.prop that anybody would be likely to change would be ro.sf.hwrotation to force your device to be natively landscape.
So, it's here:
Code:
/system/framework/framework-res.apk
res/values/bools.xml
<bool name="config_showNavigationBar">false</bool>
I looked to replacing this with an RRO (runtime resource overlay) but it appears it doesn't like my non-system signing.
It probably will be possible to binary modify framework-res.apk so that the signing is (hardly) impacted. I just have to figure out which bit in a 10M file needs to be flipped!
Edit: Well, I figured out which bits (32 of 'em) need to be flipped.
I can get a navigation bar, but there is still the issue of getting a black background on the NavBar on some apps. There is also a weird new left and right margin that appears.