I've applied this patch with revision to put the hook where I thought it
made sense.  Attached is a modification of your dummy.c to show use of
the hook.  I didn't test it heavily, but I did check that it seemed to
work with either order of calling geqo() and standard_join_search().

                        regards, tom lane

#include "postgres.h"

#include "fmgr.h"
#include "optimizer/geqo.h"
#include "optimizer/paths.h"
#include "optimizer/pathnode.h"


void    _PG_init(void);
void    _PG_fini(void);

static RelOptInfo *
my_join_search(PlannerInfo *root, int levels_needed, List *initial_rels)
        RelOptInfo *dynamic_result, *geqo_result;
        Cost dynamic_cost, geqo_cost;
        List       *dynamic_list, *geqo_list;
        struct HTAB *dynamic_hash, *geqo_hash;
        int                     savelength;

        savelength = list_length(root->join_rel_list);

        root->join_rel_hash = NULL;

        elog(LOG, "Starting a join order search \"dynamic\"...");
        dynamic_result = standard_join_search(root, levels_needed, 
        dynamic_cost = dynamic_result->cheapest_total_path->total_cost;

        dynamic_list = list_copy(root->join_rel_list);
        dynamic_hash = root->join_rel_hash;

        root->join_rel_list = list_truncate(root->join_rel_list,
        root->join_rel_hash = NULL;

        elog(LOG, "Starting a join order search \"geqo\"...");
        geqo_result = geqo(root, levels_needed, initial_rels);
        geqo_cost = geqo_result->cheapest_total_path->total_cost;

        geqo_list = list_copy(root->join_rel_list);
        geqo_hash = root->join_rel_hash;

        fprintf(stderr, "GEQO cost: %f  Dynamic programming cost: %f\n",
                        geqo_cost, dynamic_cost);
        if (geqo_cost < dynamic_cost)
                root->join_rel_list = geqo_list;
                root->join_rel_hash = geqo_hash;
                return geqo_result;
                root->join_rel_list = dynamic_list;
                root->join_rel_hash = dynamic_hash;
                return dynamic_result;

        join_search_hook = my_join_search;

        join_search_hook = NULL;
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