Hi Phil,
You can create a csv file with say email address and coupon code. Have this file populated with customer email address and coupon code on each row for 100k leads.
Then import this file in Marketo mapping email address and coupon codes to respective fields.
In your email, you would merget the field where you stored the coupon code and send that email to these 100K people.And you are done.
Make sure you test it first by adding yourself a coupon code and sending email to yourself.
Rajesh