Is there any reason that it could not be insured on a standard insurance policy? Most track/racebikes started out life as road bikes with a v5 and reg no. so why could it not be added on to a policy along with your road bike? You could even insure it as tpft as it's not actually going to get used on the road. My insurer, ebikeinsurance.co.uk, lets you make a note of any modifications, so you could put on race can and modified bodywork, etc. Admittedly it doesn't cover the bike for competition use but you aren't trying to cover it for accident damage, just theft.
Anyone any thoughts as to why this would'nt work.