custom-log.js

function customLog(v, i) {
    var args = [v];
    if(i != null && i.customParams != null) args = args.concat(i.customParams);
    console.log.apply(console,args);
}

index.html

<script src="custom-log.js"></script>
<script src="main.js"></script>

Main.hx

class Main {
    static function main() {
        haxe.Log.trace = untyped customLog;
        trace("A", "b", {a: 123});
    }
}