I am thankful we got help and were not blocked from getting SPI communication with HSD and LSD chips working:
https://community.nxp.com/t5/NXP-Model-Based-Design-Tools/Request-for-HSD-LSD-MSDI-Communication-Exa...
Longer term, it would benefit the community to have the complexity of the interface encapsulated in LSD/HSD/MSDI/TPL chip-specific blocks for MPC5777C in the toolbox we could drop into our model, knowing they would just work. Otherwise, every team has repeat this work, including the learning curve.
Even now, though I am able to send SPI messages to the chip, I still do not understand how to get status and other SPI message output from the chip. Perhaps well written C code would have to be part of the implementation for these blocks to work smoothly. It turns out getting two-way SPI communication to work well is hard.