That would require significant changes, and I doubt it can be done easily.
See these excerpts from
* Index tuple header structure
* All index tuples start with IndexTupleData. If the HasNulls bit is set,
* this is followed by an IndexAttributeBitMapData. The index attribute
* values follow, beginning at a MAXALIGN boundary.
* Note that the space allocated for the bitmap does not vary with the number
* of attributes; that is because we don't have room to store the number of
* attributes in the header. Given the MAXALIGN constraint there's no space
* savings to be had anyway, for usual values of INDEX_MAX_KEYS.
typedef struct IndexTupleData
ItemPointerData t_tid; /* reference TID to heap tuple */
* t_info is laid out in the following fashion:
* 15th (high) bit: has nulls
* 14th bit: has var-width attributes
* 13th bit: AM-defined meaning
* 12-0 bit: size of tuple
unsigned short t_info; /* various info about tuple */
} IndexTupleData; /* MORE DATA FOLLOWS AT END OF STRUCT */
* t_info manipulation macros
#define INDEX_SIZE_MASK 0x1FFF
#define INDEX_AM_RESERVED_BIT 0x2000 /* reserved for index-AM specific
* usage */
#define INDEX_VAR_MASK 0x4000
#define INDEX_NULL_MASK 0x8000
The limit you are hitting is
INDEX_SIZE_MASK, and to increase it, you'd have to change the tuple header so that
t_info has more than two bytes.
Perhaps it is as simple as that, but it might have repercussions in other parts of the code.