Getting the length of varlength data using PG_DETOAST_DATUM_SLICE or similar?

Hello, could anyone tell me, for a user contributed variable length data type,
how can you access the length of the data without pulling the entire thing from
disk? Is there a function or macro for this?

As a first cut, I tried using the PG_DETOAST_DATUM_SLICE macro, but to no avail.
grep'ing through the release source for version 8.1.2, I find very little
usage of the PG_GETARG_*_SLICE and PG_DETOAST_DATUM_SLICE macros (and hence
little clue how they are intended to be used.) The only files where I find them
referenced are:

doc/src/sgml/xfunc.sgml
src/backend/utils/adt/varlena.c
src/include/fmgr.h

I am writing a variable length data type and trying to optimize the disk usage
in certain functions. There are cases where the return value of the function
can be determined from the length of the data and a prefix of the data without
fetching the whole data from disk. (The prefix alone is insufficient -- I need
to also know the length for the optimization to work.)

The first field of the data type is the length, as follows:

typedef struct datatype_foo {
int32 length;
char data[];
} datatype_foo;

But when I fetch the function arguments using

datatype_foo * a = (datatype_foo *)
PG_DETOAST_DATUM_SLICE(PG_GETARG_DATUM(0),0,BLCKSZ);

the length field is set to the length of the fetched slice, not the length of
the data as it exists on disk. Is there some other function that gets the length
without pulling more than the first block?

Thanks for any insight,

--Mark