How to send Participant-Specific information via CiviMail
OK, so this is a partial answer to my question - don't use CiviMail but use Scheduled Reminders instead - a reminder about a specific event will have access to tokens such as event summary, event start date, etc. so I can send a reminder to those signed up for an event and use tokens to specify the event. The reminder can be reused for other events without changing the body.
That's the gist, here's more detail
So there aren't tokens specific to events available from CiviMail (except that the Fuzion Tokens Extension has lots of things that are close - such as a list of upcoming events, etc., and it adds tons of tokens about latest membership start, end, fee level, etc. THANK YOU EILEEN, and @petednz for pointing me there)
However Scheduled Reminders DOES have information about events. It doesn't necessarily have tokens for some custom fields that we use during registration, but it does have lots of information as detailed below. So whereas I can't seem to get a list of attendees for an event and then email their particular registration, I CAN cause an email to automatically be sent to attendees of a particular event via scheduled reminders.
https://docs.civicrm.org/user/en/latest/email/scheduled-reminders/#tokens-available-to-scheduled-reminders
Tokens available to scheduled reminders¶
In addition to the standard tokens that are available in CiviMail, messages sent via scheduled reminders can also contain tokens representing information about events, activities, contributions and memberships that the scheduled reminder pertains to. This allows you to use one scheduled reminder template for various events, membership, contribution or activity types.
In event messages, tokens such as event location will be replaced with the specific location of the event. In membership messages, you have access to each individuals expiry date, and so on. Membership and event ID tokens are also available and can be used to create links to specific pages (such as the membership renewal page).
Note that you also have access to the checksum token which allows you to authenticate people to specific pages without them needing to log in, and show forms with details already filled in.