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