If you are using some kind of proxy or mod_wsgi or similar tools it's usually their business to mount the application somewhere.
If you want to do it directly in paste#http probably the most simple way is using PrefixMiddleware, it will make your app available under the specified path.
Keep in mind that if you didn't use tg.url to generate your urls all your links will be broken. Also the application will still be available at both / and /subpath.
You can enable it inside development.ini with:
[filter:proxy-prefix]
use = egg:PasteDeploy#prefix
prefix = /subpath
[app:main]
filter-with = proxy-prefix