It would be very much an option for the BSPL.
As for any claims for "restraint of trade" That would be a non starter as the ACU have, like other national federations (FMN's) "legal standing" recognised by courts in countries where sanctioned events take place.
By obtaining an FIM or ACU (FMN) license, riders commit by contract to a specific set of rules, which typically require them to seek "start permission" from their home FMN to compete in events abroad, even those which might be sanctioned by another FIM-recognized body. Competing in an entirely unsanctioned rival organisation's championship without any authorisation is a clear breach of these terms.
Ultimately, the FIM and FMN's maintain control over the sport's structure by ensuring all participants operate within their approved framework, using the power to issue and globaly revoke licenses as the primary enforcement mechanism against rival organisations "poaching" their riders.
Of course riders not holding a license issued by a FMN are completely free to ride wherever they so wish to do so, but would exclude FIM/FMN events.