I appreciate the info. That said, I've got it sorted already. In this case, the issue was due to simply not handling the XMAP9 mode table bits which specify a non-CI mode. There's a pair of bits in each 24-bit mode table entry which specify whether that particular mode should be handled as CI or one of three direct RGB mappings.
The VC2 chip used on the XL/Newport boards, too, uses an RLE list to determine the starting X position at which to invoke a given 5-bit DID, which gets transmitted to the XMAP9 to determine which of the 32 mode table entries should be active at a given point.
