Needs new debugger ... or something

So, um, right now, debugging complex build processes is harder than it has to be:
1. The (formerly?) experimental `MSBUILDDEBUGGING` approach is extremely slow, seems brittle, has an unfortunate dependence on Just My Code, and for some scenarios requires running Visual Studio under the debugger. (It's _very_ easy to lose control of the target build process.) However, if you can manage to get to point in the build that you are interested in, it _can_ provide some quite useful information about the state of things at that time.
2. Log output seems to provide more-or-less enough information, but finding the relevant portions for a complex build, especially one with many recursive invocations, is fairly painful.

So, we could really use a better approach to debugging. Perhaps something based on stepping back and forth through some kind machine-readable of log file?