(You can long-press a folder on the external sd-card in Root Explorer and tap "Link folder" or something like that. I dont know the exact command in english. Then you can browse to your internal sd-card or data partition and insert the link there.)
It's basically the same as if you use Terminal Emulator and "ln -s".
For me it works, even though the external card is FAT and the data partition is EXT...
And yes, it does stick with a reboot.