wait...this is not a bad idea, but it is complicated to implement if you want it exactly that way (as a stationed playstation in a stational station)
the fun thing is we have this already implemented, but not using it at all ...
Right now, when you send a fleet to a planet, you send a 'concrete fleet' to a 'concret planet', well ...
Lets say now you send 3k bombers 4k figs and 10k droids (with transports) to a planet but you click a button called "convoy", then the fleet you send hovers the full system not only the planet, and you can choose what you use from the convoy to attack each planet in the system.
So you can use do some PIs or IPs and send
110 bombers 350 figs 8 transports to planet 1
130 bombers 50 figs 3 transports to planet 4
'everything' left in the convoy to planet 5 (cause you found it is a portal)
^there's a button to select everything like the one you have when sending a fleet (its actually the same button)
Well did you get the idea?? you emptie the convoy on the system, and the fleets stay stationed in the planets they take (you cannot get them back to the convoy, just recall them to main fleet). The convoy itself disappears when all the fleet contained in it have been used.
This is not u're idea SOL I know, but its something similar and very doable, since the code is already there, Ps only has to add the button "convoy" and the effects of it when a fleet arrives
And this should not be an extra (the cost of sending a convoy should be the cost of sending to 1 planet * number of planets in the targeted system) I mean no other restrictions than the cost, and available to everyone like a standard game option.