Right now custom work item controls  are not supported by Team System Web Access . Instead your custom control you will see this unpleasant message: “Unable to create workitem control ‘ControlName’”.
While this is a sad thing for us users does that mean that we shouldn’t use Web Access? No, of course not…
All you need to do is duplicate the <layout> information in your work item xml file.
Let’s do it
The first section gets a target-attribute with the value “WinForms” like this:
The second targets the “Web”:
Last thing you need to do is change the “Type” attributes of all controls in the web layout to use “FieldControl” instead of your custom control.
Review the results
If you take my Priority Control example from  it would look like this:
Windows layout (in Team Explorer) will show the friendly priority list (= the custom control):
Web layout in Team System Web Access doesn’t show the control, but instead it displays the regular field control. Not nice, but usable – and much better than the initial error message we had.