First let me clarify that I'm not a lawyer or a judge... but I do know what CBSA will and will not allow for import. As mentioned previously, we have had these magazines inspected by CBSA and there was no issue.
To the point of the question, one could certainly argue that the body alone could be considered prohibited as was the case when the referenced individual was charged and convicted of trying to import them. However, once the magazine is assembled it is no longer a prohibited device as it can be clearly demonstrated that it will not hold more than the specified five or ten rounds. The S&W M&P22 pistol is a good example of an allowable factory 10 round magazine which has an internal block. Only difference between that and ours is their block is easily removed by hand and needs no replacement parts in order to function with the block removed.
With that all said I suppose if you had the need or desire to transport the empty bodies it would be wise to add a rivet, but as long as they are assembled I see no reason for concern.