multisite shared menu

Currently I have a multisite installation of about 8 sites (sub directory multi sites - NOT subdomain). I am wondering what some thoughts are about implementing a shared menu. Essentially, I want the top level (primary links) to be each site and then the subsequent levels will pull the menu trees from the other sites.

I.e.

example.com
- test
- test 2
example.com/site1
- test 3
example.com/site2
- test 4
- test 5

Menu (all sites)
EXAMPLE | SITE 2 | SITE 3
test test 3 test 4
test 2 test 5