Lucozade does come in 250ml cans as opposed to bottles.
As much as I am a diabetic I am going to put my product designer job hat on and say this is completely unrelaistic in terms of buisness. The market for diabetics is so small it will be a niche product at best and would not be a worthwhile revenue stream for the company.
It would require a whole new set of tooling to create a new can or bottle (there is a reason all companies use the same can designs) and the liquid amount would require a small charge, working on the basis a 250ml can would be about £1.00 the amount you would have to charge for this to be appealing to the consumer would not be cost efficient... the can would cost more than the liquid.... infact to make a profit they would probably need to charge more for a non standard 100ml can than a standard 250ml one.
Your only real hope is the 150ml can designed for mixers in bars (funsize they are sometimes known as to appela to kids.... you also get them on airplanes) Coke comes in this size.
If you really do not want to throw lucozade away my suggestion is buy in bottles and get one of these:
http://bit.ly/1qPdCI3
Cause 100ml cans is never going to happen