Figured out what I was doing wrong and how to make this work.
In the default UI folder is eq2ui_inventory_bag.xml and the Location you need to alter is near the top:
<Page Activated="true" eq2usescomwndcontrols="true" Location="4,110" MaximumSize="1024,1024" MinimumSize="20,20" Name="Bag" ScrollExtent="225,268" Size="225,268" UserMovable="true" Visible="false">
You can change that Location="4,110" it's in the form x,y from the upper left corner of your screen.
However the mistake I made is leaving it in the Default folder. It will be overwritten there, seemingly even if you change the value then mark the file Read Only.
What you have to do (even if you don't have a custom UI but just want to make this change) is make a custom UI folder (instructions in the FAQs about installing custom UI all over this board and site) and put the modified eq2ui_inventory_bag.xml in it. Then once you do the things you need to do to activate the custom UI, the game will start to use the special version of the bag xml in your folder and it won't be overridden.
The key is, think of this as a Mod. Even if you change one value in one xml file, it is a Modification. The game will reject modified files unless forced to accept them.