Hi Dave,
I would recommend you to use CommandLineGenerator, available when you download the RuntimeLibraries SourceCode from Customer's area. With this, you can automate the generation process and maybe experimet some speed (http://llblgen.com/TinyForum/Messages.aspx?ThreadID=8331&StartAtMessage=0뗀).
Using "Participating object subset" wont work because there're tasks that must be executed always (PersintenceInfo, for instance).
You also could create a custom preset where you disable tasks that are not needed (or not needed on every generation process). For instance: ValidatorClassesGenerator task.
Also, in a large database as yours, it's often the case that there are groups of tables, not one big schema, so create one .lgp file (LLBLGen Pro project) on each table group. Of course having relations between them is hten not possible, so you have to make sure this is something you can use in your case.
How much it takes to generate code? (and how much is much?
)
(Edit)
I made a mistake, the CliGenerator is in the SDK. The SDK is downloadable from the customer area (Downloads for v2.6 -> Extras -> LLBLGen Pro SDK).