If the time of initialization is unknown and the meter will never reset, set to UNIX epoch 0:
homeassistant.util.dt.utc_from_timestamp(0)
{{ 0 | timestamp_utc }} is 1970-01-01 00:00:00, this should work too.
I stand corrected: as @balloob said, customize comes after internal conversion to ISO8601 has already been made, so this works: {{ as_datetime(0|timestamp_utc).isoformat() }} → 1970-01-01T00:00:00
UPDATE: the above works for the scope of last_reset, but in order to be offset-aware, timezone has to be included, so in case you need it:
{{ as_datetime(as_datetime(0|timestamp_utc)|string+‘Z’).isoformat() }}
Output: 1970-01-01T00:00:00+00:00