Feature request: PayPal Button form filter

We have a single PayPal account that we use for multiple e-commerce stores/applications, and we need a way, in PayPal, to identify which site/application the transaction came from. For cart-based applications, we can do this by careful construction of SKUs and invoice numbers, but PayPal Buttons are generic and otherwise offer no identification of their source.

I'd like an action I can hook to emit extra fields (such as on0/os1) into the PayPal Button form. The obvious place for this is paypalexpress::common_button_fields().

If you accept them, I'll happily submit a patch that does this, and I'd rather not have to patch the plugin sources every new release you make.