I haven't found any scripts that could automatically detect this (I've looked at the source code for Wikipedia, and they also go by browser name), which I find curious, since most sites seem to recommend feature detection as opposed to browser detection. I would find it weird if accesskeys would be the exception to that.
Also, according to Wikipedia, there's a huge load of different key combinations for accesskeys, so I don't think making an array of combinations and picking the browser's combination would be the best solution for that.
This may not work across all browsers/versions that you care about, however, in which case you will need a manual browser-sniffing-and-branching fallback. 10–20 such combinations are not a "huge" amount IMHO, and the only reasonable recourse.