crosstab/repivot...any interest?

Hackers,

We have a strong need to make a variant to the crosstab interface so
that data that is pivoted one way would be sent through a crosstab
like function so that it would be pivoted another way. For example,
if you had

row 0: a1, a2, a3, k1, c1, c2, ...
row 1: a1, a2, a3, k2, c1, c2, ...
row 2: a1, a2, a3, k3, c1, c2, ...
...

where 'a' columns are uninteresting attribute columns, 'k' is the
dimension we want to pivot on, and c1->cN would be stacked vertically,
so that we'd end up with,
row 0: a1, a2, a3, c1, k1, k2, ...
row 1: a1, a2, a3, c2, k1, k2, ...
row 2: a1, a2, a3, c3, k1, k2, ...

There are various SQL level approaches to this but they tend to be
imperformant with large datasets so that I think a crosstab-like C
implementation ought to be able to do better (or at least I hope so)
since you have to cross product rows and columns in such a way that
you can get a clean join. Cribbing from tablefunc.c I don't think
this is a terrible challenge to do in hash table style.

Questions on the table:
*) Has anyone done anything like this or know of any current implementations?
*) Would there be any interest in expanding tablefunc along these lines?

thanks in advance,
merlin