We can look at this run from the perspective of this movie:
[802] SNES Biker Mice from Mars "final round" by Baxter in 05:12.62
It uses a password to fully upgrade the bike and to skip to the final round at the highest difficulty. It was accepted because it was very entertaining. And it was not a vaultable branch (not any% or full completion).
We have this rule:
http://tasvideos.org/MovieRules.html#WeAllowPlayingUnlockableContentUsingInGamePasswords
So in principle, it's allowed to unlock hidden stuff with cheats or passwords, it just also has to be entertaining enough for Moons as a side branch, Demonstration kind of goal in the case of this run and Biker Mice.
But playing only the first level is not allowed anymore:
http://tasvideos.org/MovieRules.html#MovieMustBeComplete
So from all of the above, I conclude that if you use a password to deliver you to level 5 with this same hidden bike, and you make the movie really entertaining (I haven't watched this one yet), it would be your best chance to get published.
Whether this bike provides for entertaining gameplay we'll find out with this submission. If it does, I think you'd only need to reapply the same ideas to a run of level 5 (increasing the difficulty in any available way, while also keeping the most upgraded bike).
Otherwise, an optimized run with no passwords that just plays through all the levels would be acceptable regardless of how entertaining or boring it will be, since that would be an any% category, therefore vaultable.