I gave this a bit of thought after my first post, and it might be possible to fudge a reloading function in the game through careful SEXPing and campaign file hacking. It gets quite complicated, but I'll try to explain my idea (note, I have no idea if this would actually work. To my knowledge it has never been tried).
Into each mission, you could add events triggered by a combination of keypresses (using the "key-pressed" training SEXP), which upon triggering will instantly end the mission and send the player back to a certain point in the campaign. I haven't quite worked out all the kinks yet, but it would go something like this:
In mission:
* TriggerReload
-op every-time
-op key-pressed
# L
-op modify-variable
# ReloadCheck(0)
# 1
-op key-reset
# L
op modify-variable
# MissionTime(0)
-op mission-time
-op training-msg
# "Press A to return to the first mission branch, B to return to the second mission branch, or X to abort. Automatic abort after 5 seconds"
* TriggerReset
-op every-time
-op or
-op and
-op key-pressed
# X
-op =
# ReloadCheck(0)
# 1
-op and
-op <
-op +
# MissionTime(0)
# 5
-op mission-time
-op =
# ReloadCheck(0)
# 1
-op modify-variable
# ReloadCheck(0)
# 1
-op key-reset
# X
* ReloadBranchA
-op when
-op and
-op =
# ReloadCheck(0)
# 1
-op key-pressed
# A
-op end-mission
* ReloadBranchB
-op when
-op and
-op =
# ReloadCheck(0)
# 1
-op key-pressed
# B
-op end-mission
First event triggers the reload sequence, second one cancels it due to player choice or a 5 second timeout. The last two events simply end the mission on a given keystroke, if the reload sequence is active. If campaign persistent variables are used, those would also have to be reset to appropriate values in the event that ends the mission. With this in-mission event wrangling taken care of, one can then hack the campaign file in notepad to set the next mission to be the first one in Branch A or B, according to which event was triggered. Would work sorta like savepoints in console games. In theory. Maybe. With a lot of luck. And probably faith too, if you're into that.
One major drawback I can think of is that, at the moment, there's no way to force the game to skip debriefing through SEXPs once you're in the mission, which would make it kinda ugly even though it would still work.