The answer to your question is
No, there’s no way to specify read-only access in the connection string.
Alternatives are
1. Create sql user with read permission
MVC3 Read-Only MySql Connection String
2. create views or stored procedures with permissions checking logic in them
MS SQL Grant permission to only a view
MySQL Grant a user permission to only view a mysql view
3. Implement permissions layer in your business logic
Good Luck!