We would like to filter by leads owned by active users. The field is exposed to the Marketo Sync user but I cannot find that field available to use to build my list. Any ideas?
This is not possible.
The workaround is to create fields in lead and contact and fill them out with SFDC workflows and or trigger.
Yes, add them in SFDC, on lead and contact, and have them populated as described above and sync them to Marketo.