I'v had this same issue, but i was using Object Model to Query items and luckily the issue was solved. It turns out you have to set MeetingInstanceId parameter either to some specific meeting instance ID, or -1 which will query all meeting data.
For a webservice Lists.GetListItems method there is the queryOptions parameter you can use to set MeetingInstanceId
<QueryOptions>
<MeetingInstanceID>
-1
</MeetingInstanceID>
</QueryOptions>
An integer value where a positive number represents a specific meeting
instance. Negative numbers have the following meanings: -3 =
UnSpecified, -2 = AllWithSeries, -1 = AllButSeries, 0 = Series. This
element is optional, and its default value is -1. Negative values
correspond to values of the
Microsoft.SharePoint.Meetings.SPMeeting.SpecialInstance enumeration.
I haven't used webservices to query meeting workspace, but you can try yourself.