Our meta model supports 'default sequences' for databases which support these, and we can utilize those when a sync is performed: we can set these when a sequence is defined on a mapping. Looking into it, it seems a good practice on postgresql to have a default sequence defined in the form of the default value if a sequence is used with a table.
There's a problem however, and that is that your timing couldn't be worse, as v5.2 beta will be released tomorrow and there's no time to change things before release. So changing this (which isn't a simple one-liner, unfortunately, otherwise it would be no problem) takes considerable work on the internal code, settings, docs, tests and we can't do that at this point, so the change will be made in v5.3 (which will be after several months, likely after the summer).
That doesn't help you though, and what might be possible instead for you is to generate the alter table statements with an additional template. I don't know how many tables we're talking about, if there are just a couple, you can do it by hand, but if there are hundreds, it's easier to automate it with a template.
I'll write a template/templatebindings file for you tomorrow to generate the additional DDL SQL script to alter tables with the sequence to set the default. That will help you with this problem now so you don't have to wait till the change is made in a later version.