We have a field in Salesforce (called Mobile) that is not currently visible to the Marketo user and thus is not syncing. Does anybody have any experience with making it visible and the downstream implications? Such as: is there a huge backlog with updates? Does this tend to mess with the syncing process and/or programming?
If the field becomes visible to the sync user, this will not by itself cause all of the values in the field to be synched down and cause a backlog. These values will only be synched when there are updates to the lead record, so you can do a mass update through the data loader on your records for those that have a non-empty value in this field, which will cause all of the records to have their values synched to Marketo. Depending on the number of records which will be synched, this may cause a backlog, but it is dependent on the number of records which need to be updated.
Not sure if your first question was answered here. In order to make the custom field visible in Marketo, you must ensure it is Visible (also, editable) to the Marketo synce user in SFDC.
In SFDC, click setup. In the left-hand column in the "Build Section" click Customize>Leads>Fields. Locate your custom lead field in the second section on this page and edit the settings. (click edit) On this page you may have to click "Change Field Type" in order to enter the 3-step process for creating editing lead fields. Pass through eacho fthe steps (using the next button) to get to Step 3 where you can adjust Field Level Settings. Set this custom field to be Visible to your Markteto sync user tand you're done!
From here you can use the Field Management section to choose setting to this field in the Marketo Admin.