Even possible for not custom roms...
I yesterday mentioned to open a thread today, desribing
how to backup
your savegames, but you where a bit earlyier
But I found out, how it works with every interop-unlocked
device (samsung and htc) and even with non custom-roms. You have
to download Windows phone Power Tools
which allows you to read and write apps isolated storage
(but only from sideloaded homebrew apps) and
wp root tools 0.8 by heathcliff@xda. Also you have to download or
sideload and homebrew app, for exapmple MPA Tool, (which is also an useful app
I you dont want this, you can choose any other homebrew application
on your phone and find out its installation path.
Now, if you have wp power tools and wp root tools "by heathcliff"
there are two steps to go:
1. (wp root tools)
you have to navigate to the path of the game
(f.e. angry birds)
and copy the two files (for other games it can be more than two)
in the isolated storage of your choosen homebrew app (e.g. MPA TOOL).
(I created folders in this path to hold it clear...)
2. windows phone power tools
When this is done connect your phone with zune and run wp power
tools. Under "file browser" you can open the isolated storage of MPA
Tool (or any other homebrew app) and copy the files from the
games to your computer.
After flasing a new ROM just revert the hole progress...
PS: I attached a list with the installation paths of many applications,
so its easier for you to find the path...