State of Debuggers on iOS 7
The build of GDB from Xcode 4.4 (Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/usr/libexec/gdb/gdb-arm-apple-darwin) can be pseudosigned with ldid and run on a 32-bit device with reasonable success. Apple no longer maintains gdb (as it being GPL would have required them to release source code for it) nor have they released any source code for anything in Xcode 5 (including lldb, and it sounds like for LLVM they are only semi-interested in contributing their ARM64 backend... so we'll have to see on that one...); in essence, we are currently "out of luck" with regards to debugging on 64-bit devices unless someone burns a bunch of time porting or writing a debugger themselves. It sounds like you got close doing remote debugging from Xcode, though: maybe someone (you?) could work on a Substrate extension to whatever is checking process ownership on the device (probably the lldb moral equivalent of gdb-server) and publish instructions on the dev wiki?
(edit:) On the remote debugging front, crash-x indicates there might be useful instructions for getting a remote lldb to connect through debugserver in the following presentation:
Information on how to get remote debugging working on iOS 7 and 64bit devices can be found in this entry: http://iphonedevwiki.net/index.php/Debugserver