I have actually solved the need for cross-platform accessible encrypted backup by using TrueCrypt and NTFS-3G drivers for Mac. I created a blank (without filesystem) images with TrueCrypt, and then formatted them with NTFS. I found that so formatted image could be accessed from Windows correctly, by mounting in via Windows version of TrueCrypt.
However, this solution is quite brittle. For instance, if you forget to dismount the image properly, the NTFS metadata may be corrupted and next time you'll be able to mount them in read only mode. This is fixable by mounting it in Windows again and running chkdsk on it, but as it happened fairly often, it was rather inconvenient.
For this reason I went back to Mac sparsebundle, which comes free my home directory on a Carbon Copy Cloner-made backup. This has the advantage that my backup drive can also serve as bootable backup -- it's HFS+ at the root. In the catastrophic event I lose all my Macs and need to access the backup from another O/S, I'll probably boot into a "Hackintosh VM" or something.