The InternalMessageQueue now collects messages in a overflow queue as soon as PostMessage() returns ERROR_NOT_ENOUGH_QUOTA.
However this still means that JUCE maxed out the Windows messages queue leaving no space for further messages for anyone to post. At least some hosts such as FL Studio also seem to post messages while creating a plugin which then fails. The hosts do not seem to have a fallback mechanism (which is understandable because why would thousands of messages be queued under normal circumstances) and as such they can end up in a broken state.
I did a quick hack that fixed the issues and I suggest to implement something similar in the official codebase: As soon as 1000 messages have been send using PostMessage() all further messages are put into the overflow queue until the overflow queue is being dispatched.