使用golang.org/x/text/message格式化big.Rat

The package golang.org/x/text/message allows us to format numbers using national formats:

const n = 1222333.444555

prEn := message.NewPrinter(language.English)
prEn.Printf("%20.6f
", n)
// Prints:
//     1,222,333.444555

prRu := message.NewPrinter(language.Russian)
prRu.Printf("%20.6f
", n)
// Prints:
//     1 222 333,444555

Can I use it with math/big.Rat? That is, something like (doesn't work):

rat := big.NewRat(1222333444555, 1000000)
prEn.Printf("%20.6f
", rat.FloatString(6))
// Should print:
//     1,222,333.444555

I know that I can wrap Rat in my own type and implement fmt.Formatter, but maybe there is a built-in way already?