Avoiding SFDC overwriting newer values from Marketo

I've run into the following situation a few times and am hoping someone might have a solution:

1. A new lead is created
2. The lead is synced into SFDC as a lead
3. Smart campaigns in Marketo run triggered by lead creation, updating various data values
4. A managed package automatically converts the lead to a contact in SFDC
5. Because the conversion happened slightly after the data value updates, and steps 3 and 4 happen between Marketo syncs, the next time a sync occurs, Marketo thinks the null values that exist in SFDC are newer, and overwrites the values in Marketo

The possible solutions I can think of are:

• Retrofit the smart campaigns to wait until lead conversion is completed
• Disable field updates via SFDC sync on some fields (though I believe this would prevent the values from syncing up into SFDC as well?)
• Create Marketo-only "mirror" fields that match the fields being overwritten. If they're nulled out, a smart campaign sets the field = mirror field

None of these are terribly appealing to me -- has anyone encountered this and discovered a better way?