Peter here from Shopify Support!
The short answer would be that no it wouldn't be possible to offer a discount based on the payment processor used. The discount field requires the customer to fill in the discount code before initiating the final step of the order process. This allows the checkout to send the total value to the payment processor through the API so that it can charge accordingly. For this reason, it wouldn't be possible to get offer a discount based on payment method.
What you could consider doing is refunding a percentage/dollar amount on the orders that come in through Bitcoin. This would be similar to a "rebate" offer as you could advertise that if the customer pays through Bitcoin, they will receive a certain amount back after the purchase.
If there's anything else I can help with please don't hesitate to reply back and I'll happily help out!
All the best,