If I am heading to a Garrison contract, I just say the parts are there waiting on my arrival. If, on another type mission.......raid, assault etc, the parts are waiting on me at the LAST planet of my employer (my staging base), before entering enemy territory, for my arrival. Same for pirate hunting, closest planet of my employer is the location of my base of operations for that mission and that is where all my parts will arrive at.
It is just easier for my thinking, than parts arriving will nilly all along my jump route. Since the current MekHQ cannot split units up, you deploy entire force. I head wave that a certain amount of my force is on the closest employer planet with all my dependents etc. Since I run large units, I don't need multiple regiments hunting pirates. But I rarely take those missions once I hit a certain size. My 2784-First SW units are large multi regiment in size, but I can head wave that the pirate force is some left over Amaris unit that went pirate after the fall.
Some day, we might be able to deploy forces to multiple planets for different missions etc. And it is on my wishlist.