I assume that Sanwas telemetry system is proprietary and is tightly locked down.

I have two rx-482 receivers. It seems that telemetry functionality is not accessible without the use of their supported in-house ESCs (Obviously for sales purposes).

I can see one alternative is to use a different receiver (rx-461 / rx-462) which supports direct sensor inputs (I assume for nitro applications).

So the question being, has anyone reverse engineered these to unlock these features? I assume its just additional data tacked on the end of the PWM signal... Hopefully not digitally compressed or encrypted Though I'm no electronics engineer.

Or am I just going to have to get another receiver?

Alternatively, what other 'on-board data logging solutions' are available?

You need sensors to collect the data, and you would have to be able to connect into their system. You might be able to make an Arduino do all that, but it would probably be a lot of work. Tekin esc's have data logging. You could find a stand alone data logger like the Novak Sentry or something from Eagletree. Best bet is probably a new receiver, and the sensors to go with them.

What do you want to do with this data?

Looks like the Eagletree would do the job. Though the price + sensor is pretty much the same as a new receiver.
The only real reason why I was interested in RPM was purely for gearing. i.e. trying to maximise the duration the motor is at peak torque, power and efficiency (maximising 'area under the curve')... but mainly tuning for peak power.
I'm not really keen on only tuning by temperature. Each motor has different thermal characteristics and is affected on ambient temperature, humidity etc. i.e. too many variables. Tuning by rpms seemed like a more direct and consistent method.
It seems the only other option would be to glue 2 magnets to my spur gear and use a hall effect sensor with some kind of custom made circuit or arduino to find the peak speed - then do some basic math. Seems like a lot of trouble for a simple problem - It seems the only other option is to go by ear or "feel" as they say, which is not scientific at all :/

How consistently can you drive? If you are good enough you can make gearing and timing adjustments, and see if your lap times change. Not much more scientific and measurable than that, and ultimately the point of making an adjustment. Motor temps are a function of the load you put on them (gearing and timing) and the ambient temperature. If it is hot where you are you can bet the motor will run a little warmer. You don't want to overheat a motor, so you have to have timing and gearing right. The size of your track dictates gearing, and timing will need to be within about a 4° window to have the motor in the sweet spot. If your motor isn't slow or hot, data logging info probably won't help make you any faster. Make a small change and see how it feels.

First question you need to answer is how good of a driver are you? If you crash a bunch and your lap times are all over the place, a slight change in gearing or timing isn't going to matter because you leave a bunch of time on the track. At that point it is better to concentrate your efforts on improving your driving.