I would like this greatly (though thinking round up seems better as it can be done as part of normal share/ETF purchase). At present the only way is to sell a whole holding and rebuy, but that costs you the spread. I also appreciate that buying at the pie level or rebalance would create fractional shares again, but it is a sadly missing option.
You could have a round up tick selection box either on the share screen, since it already tells you how many you have in pies and outside (as manually bought), and/or in the pie invest custom screen since you are allocating different amounts individually.
If the concern is added confusion to some then have an option under settings to enable round up option.
If there is elsewhere I can request this let me know.