I think you don't have to worry about 'read-only' part during iSCSI initiator setup.
- using Windows default iSCSI initiator you can connect to iSCSI target (you will need to provide target ip etc).
- You can browse to disk management and confirm if your iSCSI storage is visible. Don't INITIALIZE or DO ANY OPERATION on them from disk management. Just make sure the iSCSI storage is visible in disk management.
- Finally on Veeam console, if you wish, you can set Transport Mode for the proxy as 'Direct storage access' explicitly.
Hope it helps.
BTW the link provided by foggy is very informative.