I would like to add "json" option for additional escape for backslash character. So it will be:
access_log path format json;

The initial patch for 1.6.2 looks like:
--- ./src/http/modules/ngx_http_log_module.c.orig
+++ ./src/http/modules/ngx_http_log_module.c
@@ -67,6 +67,7 @@
time_t disk_full_time;
time_t error_log_time;
ngx_http_log_fmt_t *format;
+ ngx_uint_t json;
} ngx_http_log_t;


@@ -881,7 +882,7 @@

value->escape = len ? 1 : 0;

- return value->len + len * 3;
+ return value->len + len * 4;
}


@@ -951,6 +952,7 @@
while (size) {
if (escape[*src >> 5] & (1 << (*src & 0x1f))) {
*dst++ = '\\';
+ *dst++ = '\\';
*dst++ = 'x';
*dst++ = hex[*src >> 4];
*dst++ = hex[*src & 0xf];
@@ -1068,6 +1070,7 @@
log->script = NULL;
log->disk_full_time = 0;
log->error_log_time = 0;
+ log->json = 0;

lmcf = ngx_http_conf_get_module_main_conf(cf, ngx_http_log_module);
fmt = lmcf->formats.elts;
@@ -1087,6 +1090,7 @@

ssize_t size;
ngx_int_t gzip;
+ ngx_int_t json;
ngx_uint_t i, n;
ngx_msec_t flush;
ngx_str_t *value, name, s;
@@ -1189,6 +1193,7 @@
size = 0;
flush = 0;
gzip = 0;
+ json = 0;

for (i = 3; i < cf->args->nelts; i++) {

@@ -1255,6 +1260,16 @@
#endif
}

+ if (ngx_strncmp(value[i].data, "json", 4) == 0)
+ {
+ s.len = value[i].len - 5;
+ s.data = value[i].data + 5;
+
+ json = 1;
+
+ continue;
+ }
+
ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
"invalid parameter \"%V\"", &value[i]);
return NGX_CONF_ERROR;
@@ -1267,6 +1282,10 @@
return NGX_CONF_ERROR;
}

+ if (json) {
+ log->json = 1;
+ }
+
if (size) {

if (log->script) {

But I can't figure out how can I pass the json variable directly to the functions:
* ngx_http_log_variable_getlen
and
* ngx_http_log_variable

Can someone give me suggestions?