For those still interested in editing apks on OS X without unpacking and hence without resigning I have found something that works!
! Just rename the .apk to .zip and browse it using muCommander just like you would using WinRar! Just tested it, I was able to add and remove files without resigning. The modified apk installed on my phone without a problem!
As for editing xml values you can simply copy the xml file out and edit it using your favourite editor and then copy them back in.
Hope I was able to help some OS X users.
/edit By the way, this method (WinRar or muCommander) also works for editing update.zip. This means you can customise the rom (add/remove system apks, change the boot animation etc) before you flash it.