You can, but it isn't exactly recommended. The problem with doing updates of this sort is that when you do, you're bypassing any of the application logic you may have defined in Remedy. eg, if you have a required field, or if you're checking the format of a field in a filter or something of that sort, that is all enforced by the application server, not the database. So, you have to ensure that you are following any rules that are in Remedy with the data you write directly, otherwise it will cause problems.
As for writing to tables, instead of using the real tables (which start with T, and have field names of C + the field ID), use the views that are there in the database. They'll be named just like the schema in Remedy, but with underscores replacing spaces and x's replacing any special characters.
HTH