Just in case anyone is running into the same issues, here's a quick update.
I played around with the i386-DVD-Medium of Fedora 11 and QEmu, having earlier created a container file with partitions for /boot, /, swap and an extra partition. The latter I encrypted with blowfish (using Knoppix) and put some test file there. Then I installed F11 via Anaconda, ignoring the extra partition that, as mentioned, couldn't be read. For the root partition, I explicitly gave a different password than for my extra partition.
After the installation, I used QEmu to boot a grml live cd (since I needed support for both cryptsetup and ext4) (*), opened F11's root partition from there (formerly having taken care not to make it a LVM volume) and edited both /etc/crypttab and /etc/fstab to include my extra partition in the boot process and mount it.
It worked. So at least I now know it's possible to access my existing partitions, if I should really do the switch. I still haven't found any way to supply options for how Anaconda is encrypting new partitions, though. If anyone has a pointer on that, don't be shy.
*) (this is off topic) I had to do this because I wasn't able to work in any decent manner under the standard base installation of F11. OMG, Gnome is the most unstable and buggy desktop environment I have ever seen. Ever. It makes Win95 seem a rock solid environment for the power user. I sincerely hope for all Gnomers that this was only caused by the emulation I used; I for one will stick with KDE anyway (which ran very well from the F11 KDE live cd btw). Just needed to get this off me for losing so much time - please don't make this a KDE vs. Gnome thread now.