I saw this in the SOKP thread. Not sure why that is necessary as that option is actually a part of dev tools at least in every rom I've seen to date. I'd first try Settings>Developer Options though i believe the issue is that Strict Mode is persisted and adding or editing buil.prop will be needed. Worth a try. Let us know if you can here and/or on the SOKP thread if you can post to dev forums if the dev options is all it takes.
Also many file explorers will allow you to view certain root folders, typically the root ("/") of the phone only but will not allow any write operations until several conditions are met. Of course you have to have working full root and SuperSU/Superuser installed and supersu binary updated for one. Second many explorers leave device root, /system (where your build.prop file should be) and pretty much everything other than internal and external storage, mounted -ro by default. Take for example CM's Filemanger. CM builds (official at least) come with supersu and Superuser injected in the build. This of course makes the 1st step i listed unnecess. However upon first opening CM File Mgr, access to the root of the device is restricted and no operations can be performed by the user directly at the root level. This is of course easily solved by going into the app settings and changing the mode that it operates. By default to prevent dangerous & unintended operations to sensitive files and partitions, CM File Mgr defaults to "Safe Mode". As far as File Mgr goes you have two other choices "Prompt" and "Root" modes that will allow you write access to things like build.prop at which time you will get a superuser popup to grant or deny root privledges to the app. And Just because you have recieved the superuser popup does not mean your file explorer/root browser is operating in Root mode. I recommend using root browser by jrummy,or ES File Explorer. ES is an example of a browser that requires you to elevate its privileges, root browser does not and they are both widely used.
Now I know that seems like a long answer but the idea is the same whether it CM's File Mgr or the two I just mentioned, just a little insight to common issues. This is not a fix all. Partial or buggy SELinux/SEAndroid builds will require more attention as /system stays mounted -ro except when commanded by root level apps invoked through busybox and The SELinux Contexts and other properties determine who gets access to what. Dont worry you don't have to know what any of that is just hope this helps.