Hmm, well I'm sure it's probably similar to any other js library. But you can do it with events. One number for the significant digits and one for the exponent. Then just some functions to do math between them.
Possible example.
dropbox.com/s/n7vzo5y4q9mdhzz/bigNum.capx
Something like:
bigSet("2e1000")
bigAdd("9e1000")
will result in "1.1e1001"