Backup to tape with one program and restore to a different OS with a different backup program?
This will take some serious research, but my initial response is: Not a chance.
There is a faint possibility that if you use Retrospect Mac and Retrospect Windows, same version, that you will be able to read the archive.
Tape is the wrong medium for transfer here.
You can transfer via Ethernet network with file sharing, or push the files to a server that is accessible by both machines, or burn to CD/DVDs, or get a Firewire/USB external hard drive, format it FAT32, and use that to transfer.
There is a little utility called NameCleaner which with batch-clean Mac filenames and substitute legal characters for the DOS-illegal ones, plus append three character file extensions to files based on the File Type and Creator. The big issues are slashes and periods - if you have a file named "MyLetter2\10\2006" for example, the DOS based OS will interpret that as directory Myletter2 subdirectory 10 subdirectory 2006. The file *will* transfer, but it will become inaccessible -- ("corrupt" is an adequate synonym here). Misplaced periods in the name can confuse the OS into trying to open the document with the wrong application.
There is also an issue with the path and filename length. If the total path from c:\Username\My Documents\..\..My extra long Mac filename here.doc exceeds 255 characters, the documents will be inaccessible (they might fail to transfer in this case I can't remember)
And of course, you have to have appropriate programs on the Windows machine to open the Mac filetypes. A Pages file won't open in Word or anything else. Renaming an Appleworks WP document to .DOC will not make it Word compatible. Fonts are going to be different. You have to plan for these issues in advance, perhaps by saving documents as PDFs for reading.