After doing a bit of googling, I managed to find a solution which does do its own resizing. It is here for anyone in the future who reads this topic and wants a solution:

http://www.developerfusion.co.uk/show/1787/1/

It actually works really well! I just put my one big panel in the docking control, set the panel's Dock to Fill, and then I can drag the docking control around and it works like I was hoping. It has to dock on a Form, though... you can't dock a control within a control (like a Panel), so this may limit its use, but it does the job for me.

Right, the MDI children have to be in the MDIControl. This is the control that has the gray background in the MDI parent and hosts child forms, but you can't select and modify it in the designer.

You can, however, access it in the code, as it is a member of the MDI parent form's controls collection.

Perhaps it would be possible to programmatically remove the mdicontrol from the form, add a splitcontainer, and then place the mdicontrol inside the left panel.

This is undoubtedly a big can of worms.

I'd suggest using a panel on the right and creating the for resizing yourself. It is a bit of a pain, but so will be the above.

After doing a bit of googling, I managed to find a solution which does do its own resizing. It is here for anyone in the future who reads this topic and wants a solution:

http://www.developerfusion.co.uk/show/1787/1/

It actually works really well! I just put my one big panel in the docking control, set the panel's Dock to Fill, and then I can drag the docking control around and it works like I was hoping. It has to dock on a Form, though... you can't dock a control within a control (like a Panel), so this may limit its use, but it does the job for me.