Custom Field associated with subscription/plan?

Is there any way to have one (or more) custom fields associated directly with a plan for a user/subscription?

Basically, to have the ability to select a plan and then have a field where the user can enter in some text (i.e. a URL for support with that plan).

I'm aware that I can have standard CB profile fields show/hide based on the subscriptions purchased; however, I'd like to have the user enter the text during the checkout process - so that the text values can be shown on the invoice.

Is there a way to do that that I'm just missing?

however, I'd like to have the user enter the text during the checkout process - so that the text values can be shown on the invoice.

There is no functionality for this. That can only be done via a new custom CBSubs plugin. CB Auto Actions might be able to by stringing several triggers together to do the HTML output and handle the storage, but never tried it.

Are there any docs available for creating a CBSubs plugin?
Or is there an existing CBSubs plugin that you could recommend to use as an example?

The only plugin that adds customs inputs to the basket is CBSubs Promotions and its coupon field. We've no other example. It adds the coupon form using the onCbSubsAfterPaymentBasket trigger and stores it using onCPayEditBasketIntegration so it might be entirely doable with CB Auto Actions, but again never tried so no idea.

Well, I'm game - esp. to try to do this with auto actions possibly.

I'll update this ticket IF I'm able to make it work.
(just in case others are interested)