Yes, it is indeed possible to start the game from any level. It is very simple:
At the main screen (where you first see Abe's smiling mug saying "Hello") - hold down SHIFT and quickly enter this code using the arrow keys:
DOWN, RIGHT, LEFT, RIGHT, LEFT, RIGHT, LEFT, UP
Abe will say "OK" and you will be given a listing of all levels. Select which one you want to play and press ENTER to begin.
This works for both Oddysee and Exoddus, and the variation of the level will take effect when doing this. On the Oddysee one, you can also pick which Zulag of RuptureFarms you want to go to.
Another thing, I have started to make my RuptureFarms II variation. I have nearly finished editing Zulag 2 already (EDIT: I've finished Zulag 2 now!). I'll post a couple of sneak peek game images soon.
I'm not going to bother with Zulag 1 though; just Zulags 2, 3 and 4 will be edited. I've already edited Zulag 1 in my first variation.
EDIT: I have taken some images from my variation and put them on word documents for you to download and see. See below for the download link! There are more interesting parts than this, but I don't want to give too much away!