You are right, channel pressure is not in the MIDI Remote codec for the MPD18.
I attached a quick edit of the codec files with channel pressure added, hopefully it works because I haven't had time to test.
Here's how to use it:
1. Find the files AkaiMPD18.lua and AkaiMPD18.luacodec on your harddisk
2. Replace them with the files in this archive
3. Restart Reason
Let me know if it works or not.
If you ask Akai Pro support, perhaps they will add the pressure controller to all their Remote codecs.