passing constant parameters to functional indices

I'd like to add support for specifying constant parameters when creating
a functional index, e.g. create index foo on bar (substr(baz, 1, 32));

Is this a good idea?

If so, I'd like to ask for some suggestions before I proceed any further
towards implementing it.

The arguments to the index function are represented as T_Strings. I need
to add some way to distinguish between field names and string constants.
I could create A_Const nodes for numeric or string constants, and leave
column names as T_Strings; alternatively, I could add a T_Name type and
use it to store column names. Which is preferable? What's the best way
to pass the parameters on to FormIndexDatum()?

Any other suggestions will be appreciated, since this is my first foray
into the PostgreSQL source (but it's been pleasant reading so far :).

-- ams

PS: Are mbox archives of the list available somewhere? I tried to look
for related threads in the web archive, but I quickly tired of the
"Try to produce less restrictive search query." (sic) message.