I know this is a bit off-topic, but this post was only half of my problem; I wanted my remaining tabs to show up in a specific order - a problem if there are more than nine tabs.
If you go to the workspace directory ("C:\Users\<user account>\AppData\Roaming\MySQL\Workbench\sql_workspaces\<workspace name>-<integer>.workspace" in Windows 7), you'll see files that represent the tabs. The files representing tabs that display saved SQL will be named <integer>.filename and will contain the filename of the saved SQL. The files representing tabs that display unsaved SQL will be named <table name>.scratch and will contain the actual SQL.
(I discovered the way to delete stubborn tabs from the UI was to start from the left, but you could also delete stubborn tab files here. I actually used Mike's solution.)
For tabs displaying saved SQL (at least), the tab file names determine the display order: alphabetical. For example, if there are 11 such tabs, the display order would be 1.filename, 10.filename, 11.filename, 2.filename, etc.
Using the UI to display more than nine saved SQL files in a specific order requires anticipating the tab file names (which will be created sequentially during loading), sorting them alphabetically, and using the sorted tab file names to specify the sequence in which to load the saved SQL files so that they display in the desired order, left to right. (In the example above, the tab to be displayed second would be loaded tenth.) Or you could simply rename the tab files accordingly. The latter may be the only way to sort unsaved SQL tabs.
So this does includes two answers to the original post, but mostly it's extra information for those searching to have their own way.