/system cannot be remounted as RW, even when rooted (Sony Xperia E3)
Using Total Commander to remount the / directory as RW also remounted /system as RW.

Howto:


Enable Root Functions Everywhere in TC settings.
Proceed to add a new button to the toolbar with Function Type Internal Command.
Press on the &gt;&gt; button, and select the 119 Remount option.
Press OK/Apply
Navigate to root (/) directory.
Press newly created Remount button and accept any prompts.


Edit: I've found that when I'm using Kingroot to manage root, mounting works correctly. When I migrated over to SuperSU however, I could not remount / or /system as RW.
If the above steps don't work for you, Kingroot might be worth a shot.

Another edit: While I do recommend Kingroot as an option to allow access to your /system folder, you should exercise caution with Kingroot, as it is known to gather information about your device. I would save Kingroot to be a last resort, should you cannot find any other method of remounting /system as rw successfully.