Mary,
Everyone is in agreement that it is good for organizers to post notices to users who want to receive them. Wiki-etiquette is that if someone has indicated that they are in a certain demographic, and an event targets that demographic, then it is good to send them event notices. However, this has only worked historically because there are so few events being advertised, and if it happens at some point in the future that there are lots of events then this unwritten rule may be clarified.
It is good for you to post this to project talk pages. Most project pages often do not get updated with news notices, but some are, and if you add your notice to an appropriate one then that is fine also.
To post on user talk pages you should be diligent in posting to users who have opted-in somehow. Historically, the precedent for opting in might include posting to people have made it known that they are in the event's geographical region or people who have a history of publicly participating in a relevant wikiproject. Signing up as a participant of a WikiProject is also taken as consent to receive relevant notices.
I played around with invitation templates some time ago and made [[:Template:Invite]]. I doubt that anyone has ever used it, but I used the template to get feedback on the legitimacy of spamming people to visit wp:meetup pages. What I found was that there were no guidelines in place but people liked knowing about relevant events.
Cool? Good luck.
yours,