Nominating specific parcels for sale

Is it possible to nominate a specific parcel of shares when calculating CGT after a partial sale?

I have some recent purchases ineligible for the 50% capital gains discount that I would like to exclude and all the built in rules like FIFO, LIFO, Maximise Gain, Minimise Gain and Minimise CGT are unable to achieve the desired result.

100 units @ $13 held less than 1 year
100 units @ $2 held for 2 years
100 units @ $1 held for 3 years

Sell 100 units for $20.

I would prefer to pay CGT on $9 per share ($18 * 50% discount) than $7 per share on the most recent purchase, paying a little more CGT now, saving the most recent parcel until it is eligible for the 50% CGT discount.