The maintenance costs and update contention for such a datastructure would render this completely impractical, even if consulting it were free.
Thanks for the reply.
a) Only one transaction would be updating its commit status. Its multiple readers Vs Single Writer for the position of a particular transation( a memory location ). So a reader-writer lock would reduce the contention. Moreover it releases the need for the synchronization that happens with global pg_procs now. For example something like a select which would only query the old data will never access this structure.
I am right now not able to think of anything on the maintenance cost perspective.