It means a player would be jumping across federations if they moved from Team Wellington to the Phoenix, which requires an international clearance.
The transfer window doesn't reopen until the middle of January, and Phoenix general manager David Dome said they wanted to be eligible for a dispensation in future to avoid a similar issue again.
Welsh teams that play in English competitions are granted similar dispensations when signing players.
"It's a little bit restrictive and that's why we're working with Fifa and New Zealand Football to try and rectify it," Dome said.
"All we want to do is have an understanding between those parties that we can sign New Zealand players, to clarify our situation."
The ideal scenario for the Phoenix would be that they could add School of Excellence players to their roster at short notice, rather than have to track down an Australian.
McMaster spent time with the Phoenix in 2010 as an injury replacement, and will be on the bench when the Wellingtonians host A-League newcomers the Western Sydney Wanderers at Westpac Stadium on Sunday afternoon.
The Phoenix have won only once at the venue so far this season, and captain Andrew Durante was keen to rectify that and turn the Ring of Fire back to the fortress it has been in seasons past.
"We definitely want to get back to winning ways at home," he said. "But we're not looking too much into it, we've only played three games at home at the moment. The draw came against a Brisbane team who we always seem to find it hard against, and the loss against Central Coast probably should have ended up a draw."
Sunday's match will be the Phoenix's first ever game against Western Sydney, a team with the second-best defence in the A-League.
"They're doing pretty well," Durante said. "They're obviously above us, so it's a good opportunity for us to climb above them.
"They look really hard to break down, they're compact in defence, and even when they lose games it's only by the smallest of margins. It's going to be a tough night for the front guys but hopefully we can break them down."
If anyone can crack the Wanderers' rearguard it could be Jeremy Brockie, who has scored five goals in his last four games. Durante said, with Brockie's form and Belgian striker Stein Huysegems returning from injury, the Phoenix were spoiled with striking options.
"We've got a lot of choice up front so it's nice to have," he said. "[Brockie] has been fantastic and taken the burden off [Paul Ifill] and he can just get back to doing what he does."
The match kicks off at 4.30pm.