Hey Trask Rogers, I'm in a similar boat. My company is concerned mainly with upselling one of our 20 or so products to our existing customer base (along with net news, but that isn't a huge focus for us). With the messiness things can get fun!
When a customer reaches MQL, we don't assign the lead to the account rep - we reassign to a sales rep based on product interest. We initially set product interest off of something as small as a product webpage view, but we found that product interest was changing too often and was not truly representative of interest. Now, we only set product interest off of attendance for a product-focused demo, or success in a product-focused engagement program. Shoot me a note if you'd like more details.