Can I use the Service Key or HANA SQL Access to connect to the BW Bridge Space from outside?
If yes, how?
You can access to the database schema of the BWBridge space using a "Database Analysis User" (created in System Configuration > Database Access). It is not possible to create a db user in the BWBridge space itself.
Very good question, my two cents are:
I don't think you can use RFC! I'm not sure about with Service Keys. The Service Keys seems to be a like regular CF services keys.
Technically speaking it seems SAP HANA SQL Access could be used since we know Host and Port numbers of BW Bridge; of course you should get credentials. Though as BW Bridge space does not have Database Access option that would be the way to access the data in BW Bridge space.