how do you rematch trainers in the first place?
You need the VS Seeker in Platinum. You get it from Dawn/Lucas at the entrance of Mt. Coronet.
The indication of a rematch in Generation IV is similar to that of Generation III. The initial reaction is a speech bubble containing a double red exclamation mark (!!), and the Trainers rotate in place afterwards. If a Trainer does not want to battle, they will do nothing and no bubble will appear. In Pokémon Platinum after obtaining the Vs. Seeker, Trainers can be found around various Sinnoh Pokémon Centers.
When used, the Vs. Seeker sends out a signal that allows Pokémon Trainers to find other Trainers who want a rematch. This signal affects all Trainers that are within the player's view. Once used, the device cannot be used again until it is charged. The player does this by walking a total of 100 steps. It should be noted that after using the Vs. Seeker, some Trainers may have higher-leveled Pokémon than the last battle, and their Pokémon may even have evolved.
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and the original question, can any trainers in the Sinnoh victory road even be rematched?
Nope, unfortunately. Victory Road comes under caves and therefore the trainers inside the cave can't be challenged.
The Vs. Seeker is also unable to be used in buildings or caves, even where previously battled Trainers are present.
is there a list of trainers that can be rematched?
All the trainers outside caves/building can be rematched as many times as you can. Their levels also increase depending on how much have you progressed. Iirc, in Platinum they get a higher Level boost after clearing the Spear Pillar event.
However, there's no list of trainers and how much EXP they give for Platinum.
Hope this helps!