Hi David,
I don't know of any way to do this with custom formats, etc., but you could make a cell's contents always display in upper case by assigning that cell a font that has no lower case characters. There are no such fonts in the standard Windows or Office load, but many other applications install such fonts (if you check you may find that you already have one or two), and there are also many available for free download on the 'net.
Damon