In my experience the best way to secure seats for a specific date is to book as soon as they are available! When we booked to fly LHR-JNB in J, I booked the outbound and inbound seperately through the call centre to ensure we got the dates. I believe that this also works for 2-4-1 vouchers?

If if you are more flexible, you can use the BA App for South America at least (can't seem to get Aus to work). This shows the availability on a sliding calander for the desired class of travel, which can be handy!

Alternatively if you have enough points, look at flying QR as it seems they have a great J product.