I guess it depends on the circumstances but you should know a Mosquito was protested out of a regatta a couple of years ago for not having a centre beam.
If you start beating other classes on handicap they may start asking questions about why your boat looks different from the others!

You're not the first to think whether a stub of tube or a bit of string would do instead of a beam, but the rule says its 50mm dia (or square) for the whole length and just plain common sense says a short piece of tube that goes nowhere is not a "central beam". The plans (which are still referenced by the rules) also show the beam. It's not really a loophole.

On a different tack altogether, I wouldn't consider removing the beam from my boat as I think it would just make it very difficult to sail the boat competitively. When sailing downwind with or without spinnaker we spend a lot of time in the centre of the tramp to get the hull to fly. Without the beam this would be a lot harder.

Other reasons to have a centre beam are to provide a mounting for a centre mainsheet cleat, and to stop your front beam breaking if you rig a spinnaker. There are doubts over whether the beams are strong enough to take the side loads without the centre beam on a sloop rigged boat.

As I mentioned this came up a few years ago as a serious proposal to do away with the centre beam, and it received no support at all.

As far as buying the material I know Bob Keeley in Melbourne has it (03 9555 4622). 50mm tube is pretty basic stuff that I would imagine a lot of chandleries could get. That guy in the little shop near the CYC in Rushcutters Bay springs to mind.

To summarize the voting on rule changes, everyone competing at the nationals gets to vote then at the AGM, and each state association also gets a vote for every 5 members. How the state association decides to cast it's vote on behalf of it's members is entirely up to that state association to organise for themselves.