Hi Brian,
Thank you for using Syncfusion products.
GroupViewItem renaming in FlowView mode.Your requirement can be achieved by creating Custom TextBox in editmode and setting the bounds equal to the groupviewitem in FlowView mode.
Please refer the below sample and let me know if this helps you.
http://websamples.syncfusion.com/samples/Tools.Windows/F78486/GroupView_Rename.zipRegards,
Jaya