Emilio, First, thanks for putting this together - I think everyone doing this sort of thing will benefit if we're able to agree on one upstream plugin interface.
One thing I'd like to see is support for unregistering callbacks once they are registered. For instance, you can imagine that a plugin may have 'modality', where it may care about tracing very detailed information in one mode, but trace only limited information otherwise - perhaps only enough to figure out when it needs to switch back to the other mode. I added a function to the user-facing plugin API in my own version of Pavel's plugin patchset to clear all existing plugin instrumentation, allowing the plugin to drop instrumentation if it was no longer interested. Of course, you could always add logic to a plugin to ignore unwanted callbacks, but it could be significantly more efficient to remove them entirely. In Pavel's patchset, this was essentially just a call to tb_flush(), though I think it would be a little more involved for yours. -Aaron
