Through reverse-engineering, this is what I found:

-- Meeting Id produced by the JS SDK by calling context.meeting.id: MCMxOTptZWV0aW5nX1pEWm1Nemd4TkRndE9EZ3dZUzAwTVRSakxXSm1NakF0T1dNeFpHSmpNR1JsTjJGbEB0aHJlYWQudjIjMA==

-- Equivalente meeting Id expected by the Graph API (https://graph.microsoft.com/v1.0/me/onlineMeetings/.........) MSpiMjI2NzE3Ni01MzRjLTRiZWYtODAwYS1kNTk4NjIzZTJmN2YqMCoqMTk6bWVldGluZ19aRFptTXpneE5EZ3RPRGd3WVMwME1UUmpMV0ptTWpBdE9XTXhaR0pqTUdSbE4yRmxAdGhyZWFkLnYy

If you Base64 decode the context.meeting.id, you get: 0#19:meeting_ZDZmMzgxNDgtODgwYS00MTRjLWJmMjAtOWMxZGJjMGRlN2Fl@thread.v2#0

If you Base64 decode the graph equivalent, you get: 1b2267176-534c-4bef-800a-d598623e2f7f0**19:meeting_ZDZmMzgxNDgtODgwYS00MTRjLWJmMjAtOWMxZGJjMGRlN2Fl@thread.v2

Side note: If you take the "ZDZmMzgxNDgtODgwYS00MTRjLWJmMjAtOWMxZGJjMGRlN2Fl" component and Base64 decode it, you get: d6f38148-880a-414c-bf20-9c1dbc0de7ae

The "b2267176-534c-4bef-800a-d598623e2f7f" is the user id yielded by context.user.id So I guess we have everything needed to transform a JS SDK context into the equivalent MS Graph meeting id.