Nevermind! I figured out what I did wrong. The script was correctly changing the value of the "lead.SFDC Type" token which is why in my screenshot above, it shows as Contact instead of Customer. I changed the token to "company.SFDC Type" and everything is working properly. Thanks for your willingness to troubleshoot further.
I'm thinking a workaround would be to create a new field in Salesforce that will sync to Marketo and mirror the functionality of the "SFDC Type" but will be available in segmentation smart lists.
You could have a "Newsletter Eligible" field that gets marked as true when SFDC type changes to "Customer". You would then just need to back-fill this new field using SFDC type for current customers. Once in place, you could create the segmentation you mentioned above for people with a "Newsletter Eligible" value of "true".
You don't need to segment or backfill, though. Using the JS above, the form will display correctly (and the field will conveniently be backfilled when the form is posted, though that isn't necessary).