I started looking at Royal Card (Slovakia, encrypted) on the funworld.c driver in mame. It has a preliminary status due to encryption.
Basically I started working by comparing it to the non-encrypted Royal Card (Austria, set 1).
I was able to figure out how half of the bits affect the other bits:
Dec->Enc 00 -> 22 01 -> 23 02 -> 02 03 -> 03 04 -> 20 05 -> 21 06 -> 00 07 -> 01 08 -> 2a 09 -> 2b 0a -> 0a 0b -> 0b 0c -> 28 0d -> 29 0e -> 08 0f -> 09
this can be worked out, simply enough, as:
if (~a & 0x02) b ^= 0x22; if ( a & 0x04) b ^= 0x06;
I still need to figure out how the first four bits of the opcodes, but this is proving fairly difficult since the encrypted version will not match the non-encrypted version once it has been decrypted.