@MapKey is not designed for using Embeddables in maps, so @MapKeyColumn should be used. However when using the following annotation you get a "Repeated Column in Mapping" error:

@ElementCollection
@CollectionTable(name = "TRANSACTIONS_EVENTS",
joinColumns = @JoinColumn(name = "TRANSACTION_ID") )
@MapKeyColumn(name = "type", insertable=false, updateable=false)
@MapKeyEnumerated(EnumType.STRING)
private Map<EventType, TransactionEvent> events = new HashMap<EventType, TransactionEvent>();

Also see: https://hibernate.atlassian.net/browse/HHH-5393