I've logged an idea in the community, please go upvote:
https://community.marketo.com/MarketoIdeaDetail?id=08750000000IbBlAAK
We might be able to crowd-source some solution here...
I'm thinking of a solution along the lines of Daniel G (an email alias that forwards to the specific sales rep), but uses SFDC's email-to-activity functionality. Unfortunately, the out-of-the-box functionality that SFDC provides is insufficient for this purpose, so some apex coding or an appexchange app will be necessary to extend the functionality that SFDC provides.
Basically, SFDC's problem is that it only works for sending Bcc's to yourself, and doesn't accept reply-to's from outside a specified email address.
Someone(Marketo?) could create (using APEX code), an SFDC function that accepts email reply-to's and creates an activity on the appropriate record, from which Marketo could detect the new activity using the "activity is logged" trigger.
There might be an existing app that does allows this functionality, but I haven't looked.
This should actually be part of the Marketo Sales Insight package for SFDC. I'm surprised no one at Marketo hasn't done this yet, but I've logged an idea.