These will be general tips - they won't specifically solve your particular problem, but hopefully they will give you some ideas about how to figure out what's going wrong (which is what programmers spend most of their time doing).
I haven't worked with J2ME-Polish (although I do have the book), and I only use the BlackBerry JDEs to develop (mostly because I believe they work Correctly if not Perfectly). There is an Ant task called "rapc" available somewhere on the 'net (use Google to find it - BlackBerry Ant Task) which does the compile/link/obfuscate/create-COD-file operations. There's another Ant task which I don't currently use that can automate the "signing" process as long as you're using the 4.3 signature tool (earlier versions required a user to enter their password, necessitating a Human In The Loop). The BlackBerry JDEs don't "Integrate" ant (as far as I know), so you may be stuck using it in a command line.
Regarding why your Custom screen item appears blank, my suggestions go like this: launch the application into a simulator with full access to the source code, attach a debugger and set a breakpoint at the place you expect the custom screen item to draw itself. Another is to sprinkle "System.out.println( "" )" statements in the various places you expect the application to execute, including inside the paint method (this is called - by me at least - "Printf Debugging" - I tend to dislike source-level debuggers, mostly due to my days of assembly and FORTRAN programming, and debugging-by-print-statements almost always works; I have used source-level debuggers with good results, but brute-force println tends to be more comfortable for me. Your mileage may vary). "System.out.println()" will work with the debugger, but not on an actual device (unless you attach the device to your PC via USB and attach a debugger to it). On a device, you would use the EventLogger class in your code to dump statements to the device event log; you can review the log on the device by holding the Alt key down and pressing LGLG. You can also use the javaloader application (in the JDE/bin directory) to download the event log to your PC (I find this easier to read than doing so from the device).
Hope this helps!
Karl G. Kowalski
Owns a RAZR
Develops for BlackBerry
So next phone will be........an iPhone 3G!