Donation with user specified recurring billing amount

How can I simulate a recurring donation using CBSubs where the user determines how much they want to give for recurring billing? If I use a "Donation" product type in my subscription plan, I am limited to a one-time payment option.
If I choose "user subscription" product type, user does not have ability to specify the amount they want to give with recurring billing.

I need means to allow a user to donate any amount one-time or any amount recurring if that is the user's choice.

You can't have recurring donations. Best I can suggest is use a promotion that modifies the price using a field value as a negative promotion to cause the price of the plan to increase, but this currently has issues with recurring usages so it may not work for the recurring amount.

Thanks. Recurring donation support by CBSubs in future would be great for non-profit sites.