Onyango has missed the Team of Choice's opening matches of the 2017/18 season after arriving late for pre-season due to visa issues.
The Kenyan-born defender does, however, qualify for residential permit status in South Africa after arriving in the country in 2012 to join the Pietermaritzburg outfit.
United manager Quinton Jetto has confirmed that Onyango is still with the club, who hope to register him as a local and free up a foreign quota spot.
"Brian [Onyango] is still with the club,” said Jetto.
"He has applied for the permanent residence permit and we hope it will be issued soon,” he added.
Maritzburg United are halfway towards securing their first-ever cup final appearance after rescuing a 1-1 draw against SuperSport United in the MTN 8 semi-final first leg played in Tshwane on Saturday.