Hi Julia Teske. We do almost the exact same thing for our nurture campaigns at ReadyTalk. We let the reps self select the campaign and drop the prospect into one after a conversation. This is done through a shared filter.
We don't use an SFDC campaign but the process would stay similar. We created a field called Nurture Campaign which the reps can manipulate. If the rep changes the value to "Webinar", they are added to a shared filter which feeds into the webinar campaign in Eloqua. You could easily build in an additional workflow in SFDC that adds the prospect to the correct SFDC campaign when the rep sets the field as well.
I am pretty sure the campaign attribution has to be off of a some type of field. I don't believe Eloqua can simply check for SFDC campaign membership.
