In designtime, however, you can not find a reference to a webusercontrol. In other words you won't find it in the control list of the object inspector. I'm not sure that this is common logic, however the solution is quit simple.
With the function FindControl, you can make a reference to a control and set it's properties.
var wuc : TWebUserControl1;
wuc := FindControl('UserControl1') As TWebUserControl1;
If Assigned(wuc) then begin // Make sure it is there!
wuc.content := 'Got you!';
Where 'UserControl1' is the ID of the webusercontrol on the page.