On enforcing default column value, AKA "Bloody nulls"

Hi all!
In a scenario like this:

> =# create table tab1(text1 text default 'from table');
>
> =# create procedure ins_tab1(p_text1 text default 'from proc') language
sql as
> -# $$
> $# insert into tab1(text1) values (p_text1);
> $# $$;
>
> =# call ins_tab1 (null);
>
> =# select * from tab1 ;
> text1
> --------
> [nUlL]

Is there a way (apart from triggers) to force at least one of the default
values?

Thanks!