If the designers of 3e tried to incorporate all the possible "moves which fantasy heroes may do" into the combat system itself, "Combat" section of PHB would have been a one huge book by itself. Or maybe it occupied 2 or 3 core rule books.
Instead, they made many such maneuvers into feats and class features.
So, while the combat section of PHB is reasonably short, when you really want to shoot down opponent's weapon as many heroic gunman are doing, you can take Ranged Disarm feat in CW.
But it is also true that DM should better avoid simply say "No". So, I guess you can take somewhat reverse approach.
In case of maneuvers which is already covered by the core rule, specific feat often gives the PC some benefits, like,
Change the required action type (Improved Feint)
Remove AoO (many of the PHB Improved XXX feats)
Add +4 or so of Bonus (many of the PHB Improved XXX feats)
and such,
So, you may do the reverse, Climb Aboard maneuver is a free action & requires a DC 10 Climb check. And the opponent can try to shake the PC off by grapple check vs Climb Check.
So, when a character without this feat tries to do the same maneuver, you can say, for example,
It requires a standard action
This maneuver itself (in addition to the movement) provokes an AoO
The character must beat DC 14 Climb Check
Opponent can shake the character off by making a grapple check vs Climb check -4
I am not sure if this is enough to make it balanced. I guess it will need a lot of testing if you continue to allow it in your games. But when a player wants to do something not covered by the rules and DM must adjudicate on the fly, such approach may work.