Adam,
Yes, your photo shows that the holding tank is a retrofit. Compared to the original Bavaria holding tank installation, it does not give you any room to install a macerator pump and related hoses. The obvious problem is that you may end up with an even worse problem of creating a leaky install and a smelly mess.
I am enclosing a photo taken when we replaced the sail drive. On the left side you can see the hose coming down from the holding tank ( located on the wall - some 3 feet up with plenty of room below). The white macerator pump (retrofit) is on the bottom of the engine room. Unfortunately the photo does not show the hose from the macerator out.
Perhaps you could re-locate the tank to the original factory location?