As suggested by customer support I will post my suggestion here.
I tried to pay a bill, requiring me to provide the reference number (~20 digits) in the "extra info". However, bunq only supports a very short message to be included in the payment. I had to switch to my ABN account (which allows 150 characters to be sent) in order to do the payment.
Would be nice if BunQ added the possibility to add larger messages to payments.