This article is more relevant for 6.0, As I am not sure, how much Management center makes it easier to create promotions and how much effort does it take compared to Accelerator. I did work on some on POC for management center and found it relatively easier to accelerator.
Promotion engine is really powerful and Accelerator only has limited functionality exposed.In the past I have worked on loading new promotion types that are supported by promotion engine using loading via spreadsheets and also same approach can be used to load external promotions into WCS.
The promotions loader essentially has to put entries and XML into the following tables.
CATENCALCD --product promotions
It is still a maintenance struggle from what I have seen in production for a couple of years but the functionality is not that hard to implement and these promotions are visible in Accelerator and can be disabled or deleted but these could not be edited in Accelerator because they were for created for custom features that had no OOB Accelerator support.