No offence here, but I think you might be set the bar for newer visitors a bit higher than they realizes.
Actually, if one takes the OP's post word by word literally, it is not unreasonable to assume that the OP meant to CREATE a side database rather than open a drawing in a side database by calling ReadDwg() (even so, it is still possible that the drawing being read into the side database still does not have a thumbnail in it).
But I agree that if the OP make his/her post a bit more descriptive would have been more helpful.
So, after seeing you "yes" answer, I was very curious to know how, just as the OP.
According to the same post he/she made in Autodesk's discussion forum, it looks like my guess is close (creating new side database and then add some entities in it, so the database only exists in memory an no existing drawing file being read into), thus, no way to have thumbnail being created in the database.
Is there still way to make thumbnail available to the database in a case like this?