Good ideas. I think you may just need to handle the case that the literal prefix is an empty string.

For example, here is a modified mangleLiteral method in JavaCodeGenerator. I used isJavaIdentifierPart which seems more appropriate than just isLetterOrDigit.

    private String mangleLiteral(String s) {
        String mangled = antlrTool.literalsPrefix;
        for (int i = 1; i < s.length() - 1; i++) {
            if (Character.isJavaIdentifierPart(s.charAt(i))) {
                mangled += s.charAt(i);
            } else {
                mangled += Integer.toHexString(s.charAt(i));
            }
        }
        if (antlrTool.upperCaseMangledLiterals) {
            mangled = mangled.toUpperCase();
        }
        return mangled;
    }

Here is some of the resulting literals. The 2a, 7e are "*" and "~" respectively.

...
	int LITERAL_2a = 5; // <---
	int LITERAL_7e = 6; // <---
...
	int LITERAL_HL = 9;
	int LITERAL_NM1 = 10; // <---
	int LITERAL_REF = 11;
	int LITERAL_N3 = 12; // <---
	int LITERAL_N4 = 13; // <---
	int LITERAL_PER = 14;
...

Also, if the literal "*" could be converted to its decimal or hexadecimal equivalent, LITERAL_42 or LITERAL_2A. This would allow mixed letter, digit and special characters to be represented as LITERAL_ variable names. For example, "root-node" would become LITERAL_ROOT45NODE.