If the validation is done on the Emacs side, then perhaps it would be
beneficial to use the json_*_nocheck function alternatives to disable
libjannson's validation, namely json_stringn_nocheck and
json_object_set_new_nocheck.

[1] jansson.readthedocs.io/en/stable/apiref.html#c.json_stringn_nocheck

P.S. Would applying the following diff be okay? It's a simple
optimization that avoids an overflow check (the sizeof comparison is
optimized out even in -O0) in the usual case of a JSON integer fitting
into an EMACS_INT.