The Cascade Host board (MMWCAS-DSP-EVM) only supports loading the combined BSS/MSS "metaimage" binary on the AWRx devices. This is done using the ar1.DownloadBssFwOvSPI_mult() function as you called out. The problem is that you are pointing to the BSS binary, and not the metaimage binary.
I think this is confusing since we do not have a dedicated MetaImage loading function call, but rather re-utilize the BSS loading function call.
For DFP 1.2.5, after installation into the default path, the metaimage binary can be found here: C:\ti\mmwave_dfp_01_02_05_01\firmware\xwr12xx_metaImage.bin
The below lua script call would load in the metaimage from the default 1.2.5 install path:
