On a Unix box (or with cygwin (Unix for windows), a Unix environment), you could pipe the file, and get a listing out of it, or probably make a zip file of the whole thing, which would allow random access.
In some ways, tar is a feature, as it was meant for tape drives (one way linear access).
Get a Linux vm and a virtual env (say, vmware server?), or cygwin, and have fun.
7z -options_to_extract_to_console/pipe wiki9.tar.7z | tar t > listing_of_files.txt
would give you a list of what was there
7z -same_options_as_before wiki9.tar.7z | tar -xT list_of_files_I_want
and it should write them (possibly someplace weird).
I think. I don't have the listing for 7z switches on Unix handy right now.
Or to make it a zip archive:
7z -same_options_as_before wiki9.tar.7z | tar -x | zip wiki.zip -
Note that it will likely be 2-4 times larger (better to stick the files together and then compress than compress one at a time, and 7z uses better compression).
Expect it to take hours.
Consider borrowing an external drive for a few days :-D or upgrade your HD, heck under $100 for internal or external over 200gb, even laptop HD.