We realise that some people really want this, but we have been hesitant to just go ahead and implement it for a number of reasons.
For a start, we have complete creative freedom, which means we don't have to approach problems in the same way as other developers. Rally points are a trope of RTS games and we have tried in general to stay away from these when creating the game. We do not want to go ahead and implement something just because it is in other games.
Secondly, we have not experimented enough to figure out how this might affect the gameplay. If we try it and it changes the game too much, we might not agree with putting it in.
Thirdly, and this is a kind of combination of the previous two points, it might not be the right solution to the problem, which seems to be that after some duration of play, some players are finding the management of the seedlings in their empire tedious.
So, we totally get the fact that the frustration is there for some, and it is something we may be looking at in the future
In the mean time, to avoid having to wait for really slow units, I recommend you use the "ability select menu". If you double click on an asteroid, you can filter seedlings by ability. It allows you to filter seedlings with high attributes in Energy, Strength or Speed, and then select the exact number you want to send in the normal way. So in this case you can choose to send, for example, only the fastest seedlings whenever you like. That should really help, and opens up new strategies as well!
edit: I have now locked the four other threads discussing this feature; any further discussion can go on in here