[plt-scheme] getting the constructor from a struct-type

I would like to perform functional update on arbitrary structures
(modulo inspectors) and thus I'd like to be able to construct instances
of a structure given the structure-type.  However, struct-type-info
doesn't return the constructor (or the selector).  Before I write an ffi
extension to call scheme_make_struct_values, I wondered if anyone had
run across this problem before, and if there were any better solutions.

Thanks, 

sam th