It's not very pretty, but you can perform a non-case sensitive comparison by converting both sides of the comparison (or one side if the other is a constant) to lower case.
So if I wanted the filter to include rows where field1 = field2 (ignore case), I could use the formula
translate(my:field1, "ABCDEFGHIJKLMNOPQRSTUVWXYZ", "abcdefghijklmnopqrstuvwxyz") = translate(my:field2, "ABCDEFGHIJKLMNOPQRSTUVWXYZ", "abcdefghijklmnopqrstuvwxyz")
That translate() function mess is the magic that returns the lower-case version of a string value.