Here is what you should do.
1) create www/admin/index.vuh to redirect to custompackage/admin with internal redirect.
2) create www/admin2/index.vuh to redirect to acs-subsite/admin.
Overall I think the index.vuh in the filesystem is generally easier, but you might conflict with the /admin/ permissions filter if your permissions are not the same for the custompackage as acs-subsite.
You could also register a filter. but I am not sure if the request processor would register your redirect filter before the admin permissions filter.
I proposed a solution for registering filters before rp_filter, its a pretty simple fix https://openacs.org/forums/message-view?message_id=463984 but is not in the core yet. I think I'll TIP it for the next major release.
I am using this on a couple of client sites for authenticated RSS so it has been tested.