This would not work. The problem isn't a solution that requires a NEW prefix, but one that there is a ton of information and implementations ALREADY EXISTING that use the -webkit- prefix. Those devs that were too lazy or uninformed to include -moz- or any other prefixes (sometimes even the unprefixed version at all) are not going to go back and update all of their prefixes to -x- now. We don't need more prefixes.
A better solution would be for browser vendors to state that prefixes would only work in alpha/beta browsers, but would be removed from production browsers. That would allow testing, but remove the temptation to use the prefixes in production code. Alas, the standards committee can't keep up with the changes that are required for the browser manufacturers to feel safe doing so, and it could introduce the risk of features landing unprefixed that use different implementations if the manufacturers ignore the standards.
A better solution would be for browser vendors to state that prefixes would only work in alpha/beta browsers, but would be removed from production browsers. That would allow testing, but remove the temptation to use the prefixes in production code. Alas, the standards committee can't keep up with the changes that are required for the browser manufacturers to feel safe doing so, and it could introduce the risk of features landing unprefixed that use different implementations if the manufacturers ignore the standards.