Yeah, any recurring fee handler could be written to accommodate indefinite billing. I wouldn't recommend the core recurring handler for every situation anyways, especially when you're using a payment gateway that supports recurring billing. If you're using the core handler, you can always copy it and rename it in a custom module.

I think I was also having trouble figuring out how to handle the data in the DB at the time and someone else mentioned some difficulties w/ indefinite billing in the forums. Some of the trouble stemmed from the fact that the core handler is very basic and won't trap errors returned from the gateway to know what to do on failures.

Anyways, you could have your handler know that if the # of billing periods is set to 999, then treat it as indefinite. Of course, if you're collecting monthly payments, I can't imagine anyone's card is going to last 999 months anyways... well, not to mention their lives.