Is there any simple way to do the equivalent of the Commerce API's order.setCustomFieldValues(customfields) method?

At the moment the best way is to add a module to the checkout that sets this value. In the next release of SC we should be adding in an extensibility API method to do this independently

Thanks Steve. My concern is the degree to which we're having to extend the system and how that'll perform with several million pageviews a month. I think having setCustomFields methods on both customers and orders (which persisted when set before customer registration) would allow things to be a lot leaner.

Well, as I said, we're introducing an API method for setting order custom fields soon. As for user custom fields, I think that should be relatively straightforward with SuiteScript 2.0, so I don't think we would need something for that?

Customer fields are easy enough with SS2 after account creation, but I can't figure a way to store a persistent value set before registration. I tried using runtime getCurrentSession() to set values, but couldn't then retrieve the values.