Interesting -- I'd not heard of this status/function, but have observed white-clothed assistants with Burmese monks. I thought they were novices.
When U Pandita Sayadaw was last here in San Jose, a couple of years ago at Vesak, there was an American woman in his retinue who translated his dhamma
talk into English. She wore all white. I talked with her briefly, asked if she were a bhikkhuni; she said no. I didn't know to ask of "Anagarika" status then.
A Vietnamese woman at the same center, who often translates dhamma talks into Vietnamese (the monastery with Burmese monks, is in the heart of the "Little Saigon" area of San Jose, Calif.), also seems to live and work there, and often wears all white.
This video that shows Mahasi Sayadaw, ca. 1952, shows a procession of people in what looks like white robes: (at ca. 4 minutes 25 seconds in)https://www.youtube.com/watch?v=_U7TtSeOlG4