I released a new version of the Lightroom SDK 5-6 Debugging Toolkit. The debugger has been rewritten and is much more capable: breakpoints not requiring source modification, stepping, full call stacks with access to the variables of each call, stack tracing across calls to pcall() and callWithContext(); a user interface that takes better advantage of controls introduced in LR 4 and 5.
The plugin has been renamed from debugscript.lrdevplugin to debuggingtoolkit.lrdevplugin.
Be sure to read the documentation about preparing your code files -- there have been some changes (for the better).
Thanks to Zerobane for showing how to access the uncrippled Lua debug() and sethook() functions.