FKs + partial indexes?

I was thinking a bit about a problem that was mentioned in the 
discussion TomD, Simon Rigga and I had about enums and foreign keys, 
namely that often we find dozens of tiny little reference tables 
littering the data model, or else we find a table that somehow 
consolidates them, plus some sort of homegrown referential integrity 
checks. I wondered if we could improve on that situation by using 
partial unique indexes on the consolidated table, and providing a 
mechanism to specify which index the FK must refer to (or else allow 
allow an optional predicate expression which would have to match the 
predicate expression of the partial index).

So we would have something like:

create table constants (type text, id int primary key, value text);
create unique index x_const_idx on constants(id) where type = 'x';
create table client (id serial primary key; xid int references 
constants(id) using x_const_idx);

Of course, this is a blue sky idea, and I haven't thought out any 
details at all, but it struck me that it might be a way to make designs 
a bit cleaner.

cheers

andrew