osm2pqsql and lua

I installed osm2pqsql the other day. At one point it said "lua libraries not found. You will NOT be able to use lua scripts for tag transform.". I had no idea what a lua was or why I would want one, so I had a bit of a read here and here. After reading "This allows you to unify disparate tagging (for example, highway=path; foot=yes and highway=footway)" it now sounds very useful indeed **.

There's what seems to be an example here, but what isn't clear, unfortunately, is whether the lua script passed to osm2pgsql on the command line has got to implement all of what's currently in style.lua, or whether something much simpler (for example just rewriting one tag as another) is also possible?

** I've done this to Garmin maps for years - generally sticking extra info into the name tag to reflect things such as the legal status of a footpath. It would be useful to be able to do this elsewhere too.