Ok, in the documentation, under "Integrate C Functions Using Legacy Code Tool", there's a Supported Data Types table, where it says that "Array of Simulink.Bus" is supported for input and output, and for work vectors, but not for parameters. I assume the initial condition of a memory block is a "parameter". It's unclear if this applies though since it's on a page about integrating C code, and that's not what I'm doing. That is, does the "Supported for Parameters?" column on this page just refer to s-functions, or does it apply more generally?
The following question is similar and contains an answer that says that what I'm trying to do is not supported, though it doesn't cite any sort of documentation:
The solution suggested there (use a bunch of memory blocks and concatenate them manually), however, doesn't work for my application.