I only have 11 Workpads, and I do have some of them that could be grouped by "principal collection themes" too, but it's not enough to worry about right now. Of course I'm using Awasu for personal use at home, but I could imagine if I had to work with 20-50 Workpads it might get a little busy.
Here's a couple of thoughts that may make managing many Workpads easier.
Click on "Organize workpads..." and change the order of the Workpads to suite your needs. Either sort them 1) alphabetically, or 2) most frequently used Workpads on top, or 3) by principal collection themes (to add pseudo-grouping), or any combination of these sorts (e.g. alphabetically within their principal collection theme).
Also in the "Organize workpads" dialog box you can assign Hotkeys to open your most frequently used Workpads. Note to Taka... it might be nice if the assigned Hotkey acted like a toggle and would close an already opened Workpad. I'm not actually asking for this, it's just something I thought of right now.
Note: When assigning Hotkeys I usually like to press the Hotkey first outside of the "Organize workpads" dialog box to see if it already is assigned to something useful, and if so then I'll probably pick a different Hotkey combo.
If you receive more requests for better Workpad management features, you might find something similar to Channel Filters (Workpad Filters?) might be a simpler alternative to a full tree view, especially since the hierarchy is only one level deep.