There is no way to specify this in NORMA right now. This has to be done somewhat carefully as one value type can map to multiple columns, so (like value constraints) a distinction between a 'default value' on the role vs. one on the value type vs one on the data type would need to be made.
At the XSL level, we currently go from ORM+CDB (ORM with the 'Map to Relational Model' turned on) to DCIL (relational schema definition) to DDIL (DDL language definition). There is currently no notion of DEFAULT at the ORM+CDB or DCIL, but there is a 'defaultClause' at the DDIL level which should format correctly to your DB output if you can sneak it into the DDIL the file.
There is a built in mechanism with the generators called a 'Format Modifier' that I've referenced a few times on this site. Basically, you register a separate generator file that can modify each of the generated files before moving onto the next step. I've shown this used for using custom properties to specify multiple SQL Server schema names and to add audit columns to any DB (for example, see http://www.ormfoundation.org/forums/p/573/1706.aspx#1706). Both of these modifiers apply to the DCIL, meaning that the DCIL that is transformed to DDIL is not the one generated directly from the ORM file. It will likely be a while before this is designed in formally, but it would be fairly easy to write a format modifier to take a default value (based on a Custom Property from a role or value type) and modify the DDIL.
In your case, you want a system-named function (CURRENT_TIMESTAMP) instead of an actual default. I have a bit of a concern with rendering this instead of a constant value, but the system will probably just leave it untouched.
I'm curious as to the level of interest in this. The format modifier to do this would be fairly straightforward (around a page of XSLT) and could be directly based on the earlier modifiers.