This feature also greatly interests me, although I'm looking for a system that would allow to slightly alter the pronunciation of individual words. Is anyone aware of such a system?

Especially with TTS in a language other than English (but also with English), the pronunciation of certain words is sometimes jarringly wrong. Until TTS systems can compensate for this themselves, it would be great if it were possible for humans to use such tags to hint the system to pronounce better. Even if you can't specify the exact correction, but the TTS would just generate a 'different' sound, that could help.