The SAAMI max pressure for 300WM is for all intents and purposes the same as for 308. A smidge higher, but nothing to write home about.
You get your higher velocities by taking advantage of the larger case capacity. By using larger charges of slower powder. If you try to use the same powders, you won't get 300WM velocities.
So, you could try to use H335 with your lighter projectiles, so long as you keep the charge weights and target velocities in the correct range. There's plenty of data for 30-06 to guesstimate a start point.
The poor case fill comes with its own problems, though. You get the possibility of an accidental overcharge big enough to blow up your gun in spectacular fashion. And you might get erratic ignition or even unexpected pressure spikes.
I've heard it said that when using the most "appropriate" powders for your bullet weight and caliber out of a modern rifle, it's pretty much impossible to blow it up with an overcharge; wreck it, yeah, but blow it up, probably not. Not that I'd test that on purpose. Using a faster (and in the case of H335, more energy dense) powder brings some risks. And unless you want to load low velocity ammo, no obvious reward.
The liability and performance limitation are probably the main reasons why published H335 loads for 300WM are hard to find. Manufacturers could probably find a useable load with a heck a lot of powders, but I'm sure they have better things to do than to test 200 different subpar and potentially dangerous powders in every caliber. Being that H335 isn't too far off from the norm, it might be worth it for you. Blast shields, gloves, and sacrificial test guns being a good idea if you want to find out what the maximum load is. Or if you have a chrono, you could just stick with 30-06 velocities with the same bullet weights and a gradual work up. It'll probably take a pinch more powder to achieve those same speeds in the larger case.
In the very beginning, shooters didn't have a lot of choices. They made due with what the could get or make. And they adjusted, accordingly. Nowadays, hundreds of different powders are all laid out in a catalog, and the load info is cutnpaste. That doesn't mean if it's not in print, it can't be done. Just means you need to use common sense, tread carefully, and own the consequences.