"I'm thinking drill a vent into the chamber like you have on the SMLE and others of the period"
I'm not sure that would work in a gun like the Sebu.
In guns like the SMLE, the Arisaka or the Springfield there's a lot of action behind the chamber/bolt head, making vents feasible for venting gas that gets back into the action. Those vents are there primarily in the event of either a pierced primer or a case head failure in an unsupported portion of the chamber.
In the Serbu, that screw up cap is the extent of the bold head... there's literally no enclosed action behind it of the kind there is on an SMLE, et al. It also means that, unlike most bolt action rifles, the cartridge head is fully support in the Serbu.
While I'm not 100% sure, it also appears that the mode of failure in this case was overpressure causing the cap/barrel threads to fail due to increased thrust against the threads. When the threads failed and the cap came loose there was no amount of venting in the world that would have stopped what happened.
As someone else pointed out, that's not the proper thread type for high strength.