I didn't ever resolve the issue, unfortunately. I've kinda back burner'd the project in favor of waiting for the ROM SIMM programmer from dougg3. 
To work around the issue, I ended up adding my own VBL task to the VBLTask queue that ended up just calling through the low memory global pointer (0x8ee) to whatever the registered CrsrTask was. So in my case, the VBLTask queue was getting processed which leads me to assume the VIA interrupt that triggers VBL processing was firing correctly, and the registered CrsrTask was correct and working since my own VBL task was calling it, just for whatever reason the normally registered CrsrTask was not being invoked.
One odd thing I did notice in the .Sony driver was the SonyVars at 0x134 contained function pointers to various driver functions, which presumably exists as a patch point for the System to provide its own implementations.
To work around the issue, I ended up adding my own VBL task to the VBLTask queue that ended up just calling through the low memory global pointer (0x8ee) to whatever the registered CrsrTask was. So in my case, the VBLTask queue was getting processed which leads me to assume the VIA interrupt that triggers VBL processing was firing correctly, and the registered CrsrTask was correct and working since my own VBL task was calling it, just for whatever reason the normally registered CrsrTask was not being invoked.
One odd thing I did notice in the .Sony driver was the SonyVars at 0x134 contained function pointers to various driver functions, which presumably exists as a patch point for the System to provide its own implementations.