does anyone know if the "sales order" field on the customer deposit is accessible via workflow? can't seem to find the right match

I think they're related only via the 'Applied To' and 'Applying' transaction joins, which in general aren't available since they are line-level joins primarily (even though Deposits on the surface connect via the header level only). Did you check the 'Created From' just to be sure?

it wasn't an option, but I ended up finding a SuiteAnswer with a simple workflow action script that ended up doing what I needed and more!

oh nice - wf action scripts are super useful for sure