I've got codes for a couple of your games...
-Castlevania Bloodlines and Sonic Spinball:
EAAT-AY1T
Lets you play some "Non" Japanese carts on your Japanese Mega Drive.
All that code does is change the region/country ID letter in the Genesis ROM header. The only problem is that not many games use that ID letter to determine the region, so that code will only work on some games. Give it a try on your other carts and let me know which ones it works on.
EDIT:
I forgot to mention that the code above will only work if you have a Japan "NTSC" system. I don't think it will work on a Japan/Hong Kong "PAL" system. Let me know which system you have.