Would be great if this would even become part of MSD standard solutions.
Not going to work at this time. Another feature limitation in Marketo. We have attempted to perform what you are doing but backwards and have found "Source Campaign" does work but any other Lookup field does not work. We having been sending the GUID for MSD "Source Campaign" from custom webpages into Marketo using the API. We have been successful with this. Marketo accepts the GUID and will sync the GUID with MSD. MSD accepts the GUID and populates "Source Campaign" correctly. However when we attempted to apply the same approach to another Lookup field, this did not work. After working with Marketo support for 4 months here is the response:
Thank you for your patience while our escalation engineers investigated the issue. We were able to identify that this is currently expected behavior for the way that Marketo handles Many to One relationships when there is a Foreign Key (FK) reference involved. The behavior is that as long as the referred object (in this case, account) is enabled, the FK field becomes the relationship filed and we can't push the value from Marketo to CRM. What we are running into is that the FK relationship also exists in Marketo using internal IDs, thus the change for the field value in the Preferred Dealer FK field through Marketo doesn't change the internal ID which will break the FK integrity and cause the updated value not to sync. Our product management team is aware that this is a feature that customers want and are working on design plans to improve the integration to support this scenario. The solution is a large project and there were some early design testing last year that has taken place but currently there isn't an ETA on when this feature will be available.