At present there are only 2 ways you can email ALL attendees directly:

1. Convert all guests to constituents (laborious and unnecessary)

2. Export out the current event participants --> format the fields to comply with NetCommunity requirements --> import the data into NetCommunity as a single use static list.

If there is a need to email attendees multiple times, you have to export out a new list each time in order to ensure that your list is current.

I don't understand why you can't use a participant query as a viable query type in NetCommunity.