You have put the "gauge.tga" into the "images" file in your custom Ui havent you?
I guess you have, and it still dont work it is probably because of the file called boho.xml
Have a look through that there is loads of refrences to gauge etc in there
Also the eq2ui.xml file has this line in it <include>boho.xml</include> and its that what loads up the boho file .......
So I would say find out exactly what lines of code are for the gauge in the boho.xml file and take the rest out, save the new cut down version of the file and put it in your custom UI folder, Then add this line <include>boho.xml</include> into your eq2ui.xml file in the UI folder that you are wanting this to work with..
That should work... But its a bit messy... there must be a better way to do it but offhand I cant think of one lol
Other way would be to ask Boho if he would make a standalone mod for the casting window, That you could just pulg in maybe
This is just an idea to try and help you out I am no expert here, so backup all the files you are going to alter to a safe place first ! then if and when it go's wrong lol you can replace them easy