This looks like it should be a pretty simple binding to write and it does address an undeserved area (IR control).
Absolutely stick with the HTTP API for communication from the binding. Those who want to can use the MQTT binding to talk to it without need for a separate zmote binding. Plus you don't want to have to depend upon there being a broker installed.
I'd write this as an openHAB 2 binding rather than an openHAB 1.x binding. OH 2 is nearing its first full release and when that happens it will become the official openHAB and openHAB 1.x will be essentially end-of-life (though I'm certain there will be a large population of users who can't or won't upgrade).
The API looks simple enough that one could integrate it without a binding at all, instead just using the HTTP binding or MQTT binding. You might start with that to work out the use cases (i.e. what is the appropriate openHAB Item type to represent each of the controls zmote supports, e.g. represent Volume as a Dimmer, channel as a Setpoint, On/Off as a Switch, etc.).
I've not coded a binding yet so can't help with that part. Hopefully someone will chime in.