This is definitely possible live, and I have been using (as well as the more "traditional" buy on the ask + 0.05 / +0.10)
The script for a limit split spread buy order is as follows;
Route=Limit;price=bid+ask/2;price=round2;buy=send
Share position is left blank, then you can simply select the desired position within the relevant montage, or alternatively insert a pre-defined value into the script
You could also adapt the above for limit split spread Sell orders as well
This specific script has been confirmed by DAS