But Folau and the Tongans remain bullish, telling The Australian's Brent Read their plan is legally sound and they will meet any challengers head on.
Folau is taking legal action against Rugby Australia, alleging restraint of trade over his ban from Super Rugby and international rugby union, the traditional 15-a-side game.
The National Rugby League, which governs the 13-a-side version of rugby in Australia, has previously said it would not re-register Folau because of his rugby union ban.
Prominent Folau supporter Alan Jones questioned any move to oppose Folau's bid to play with Tonga.
"They're trying to stop Israel Folau playing for Tonga," Jones said on 2GB on Tuesday morning.
"Any person in any form of administration is trying to stop him playing. He must be some kind of criminal, is he? What has Israel done except profess his Christian faith?"
Folau played for the Melbourne Storm and Brisbane Broncos in the NRL. He represented Australia in rugby league before a stint playing Australian rules football and eventually rugby union.
Tonga coach Frank Endacott welcomed the potential selection of the Folau brothers for the international matches.
"These guys are great footballers and bring size, pace and skill to the team," Endacott said. "They will make a huge impact in the team and provide exciting attack options all over the park."
"This will be a great boost to the internationals, and I can't wait to see the crowd support that this will generate," he added.
The Tongan NRL statement sparked immediate headlines in the Australian media, where Folau's ban has polarised opinion.
In a social media post, Folau listed gay people among a group of sinners whom he said would face damnation unless they repented.
Folau, who was born in Australia to Tongan parents, had previously been warned after a similar post during Australia's national debate over the legalisation of gay marriage. Folau, who describes himself as a devout Christian, said he was referencing the bible.
Folau was an undisputed star in his 73 test matches for the Wallabies after joining the rugby union ranks in 2013. He has the record for most tries by any player in the Super Rugby tournament.