Sadly, Unfortunately, you are correct, no backup of mods...
The intent in AC is to always work in pln files.
If you want to edit the mod, "save as" pln file again to work on it, when you are done editing, "save as" into mod file again, that way AC backs you up as you go.
Personally I am paranoid about data safety. I create separate pln files all the time, like "MyProject-02-15-2010-2pm.pln" This has two virtues, 1) double back ups (I did say I was paranoid), and 2) a history of the project. In data management they call this the Father, Grandfather rule. If you lose the father you can always go back to the grandfather. Additionally, if you experiment in AC with design, you can use the old files to get back to a point in time where maybe the roof or elevation was different. These backups I make do not update. Imagine if you have someone come in and alter the design in a most unfortunate way such as stretching with the heavy marquee and altering another floor and never noticing until too late. (In my case that would be me in deadline panic mode.)
When I get to permit stage, I save an archive file *.pla, then copy the entire project folder to CD disk, and erase all the unused versions of the project.
Snap