When are sector sub-types determined?

Yes - sector type is determined at the start of the game

The sector layout, sector type and sector subtype is determined at the start of the game, so save scumming will not result in getting different sector sub types.

I tested this by doing the following

• Start the game
• Proceed to the end of the sector
• Save and exit
• Go to My Documents\My Games\fasterthanlight and make a copy of continue.sav
• Start the game
• Go to the next sector and check sector type
• Exit the game
• Restore continue.sav
• Start the game
• Go to the next sector and check sector type

So, I got to the end of the sector, saved and exited and backed up my game file as mentioned above before clicking "Next Sector":

I then restarted the game and clicked "Next Sector" to progress;

As you can see, the sector types shown that I am able to progress to are "Uncharted" and "Civilian". I selected "Civilian" to see if this would then generate one of the subtypes but just got a plain "Civilian" zone:

I exited the game and restored my backup save and tried again, selected the "Civilian" sector and again found that this was just a plain Civilian sector.

I repeated this process two more times to rule out the possibility of any random luck generating the same sector types repeatedly. With this in mind, it looks like the sector types are generated at the start of the game.