There are several practical solutions known but the drawbacks of choosing callbacks over hooks are already know quite well too:
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Callbacks are functions. It means that they cannot use variables from global scope implicitly. Which in turn means that plugins need to be changed to use GLOBALS or import the variables they use. Or some other trick if you know it.
Callbacks must have been processed by PHP before they are called. It means that mass-loading needs to be performed before the actual request handling is started. It needs to be quite smart not to include files that the current request processing does not require.
May the Source be with you!
