Dear Gordo,
We've followed that convention for London meetups for years, but the vast majority of attendees are from the English Wikipedia.
But when it comes to outreach events targeted at new or newish editors, it just gets too confusing to get them off EN wikipedia. There are also practical advantages, it gets them to confirmed status on EN wiki more quickly, and without global watchlists multiple wikis are a complication too far for most newbies.
More practically if you are going to have an event with a bunch of potential articles listed they will only show up as redlinks if you are on the same wiki.