Your best bet is to use Account Owner info, or Lead Owner info (depending on who you want, if they're different). Otherwise there really wouldn't be a way to know which user you want. Those Account and Lead info should be available as tokens, today.
For us, we write this information on the lead and contact record in Salesforce. For example on the Lead, based on the relationship to the lead owner we have formula fields that look up the owner's email, phone, etc etc. This is sometimes useful for other processes as well. We then make those fields visible to Marketo where we are able to reference them.
I'm not sure if you can sync with the User object in Marketo, but this approach saves you the step of having to build the relationship from the user to the lead/contact as well.