I just drove through North London (3 hrs stuck in traffic) and had reggae (courtesy of countless local and pirate stations) blasting out of the stereo the whole time... reggae is a big word, there is as much to the theory as soul, funk, jazz, blues... ska and rocksteady derived from those styles, reggae from ska and rocksteady, then came roots, dancehall, ragga, bashment... the label studio one and the producer coxsone dodd - very important. The big Bob Marley "hits" were actually re-recorded and re-produced for mainstream audiances (thats not a critisism), hence the term roots rock reggae.
I've heard the lydian scale mentioned before.
I would suggest playing I - IV progression on the offbeat as the "skank" - on a keyboard play one chord inverted then just move a finger or two to play the other. For the bass (as already mentioned) use one drops - missing the first beat - and play 3rds and 7ths along with the usual root and fifths, short repetitive and melodic.
I think sometimes that the tempos are actually quite fast eg 160 bpm but played halftime. There are some great drum styles eg rockers, nyabingy...
my favourite reggae is the digital style - check prince jammys productions 1985 - 92 (ish) - an obvious example is the notorious "sleng teng" by wayne smith - in some circles this kind of track would be called a digital killa