I think I'm missing the point here. :-)

Values created by generators shouldn't have any real meaning that just be an
unique identifier of the row. So, I just don't care if this is a number,
it's just an ID.

Maintaining 'proper' generator values are quite easy for this: just create a
generator and use that generator for all tables. A simple trigger that will
check for null values and get the generator value will be enough for allow
this value to be set either in the client and/or in the server side.

IMO, trying to make a generator ( that runs outside any transaction ) behave
according to something ( that runs inside any transaction ) is something
that just don't make sense.

Artur