pg_xlogfile_name_offset() et al and recovery

Currently in HEAD and 9.6, one can issue a non-exclusive backup on
standby, so this is OK:

select pg_is_in_recovery();
pg_is_in_recovery
-------------------
t
(1 row)

select pg_start_backup('sby-bkp-test', 'f', 'f');
pg_start_backup
-----------------
0/5000220
(1 row)

However the following happens:

select pg_xlogfile_name_offset(pg_start_backup('sby-bkp-test', 'f', 'f'));
ERROR: recovery is in progress
HINT: pg_xlogfile_name_offset() cannot be executed during recovery.

Should this restriction be relaxed or am I missing something?

Thanks,
Amit