Seems like mounting ignores the GCD after these and works anyway.

No idea how I would implement it. Seems like it would require moving at least all the mount actions and possible everything from type=spell to type=macro in the SecureActionButton. And a way to prepend to the macro before it was run. And a ridiculous amount of testing to make sure all the weird edge cases worked.

Won't be until 2021.

Also Slow Fall?

I don't think I have time to do this properly, in a way that would allow you to compose a complex macro. Best chance for immediate success is something like

PreMountSpell Slow Fall, Path of Frost, ...

have it set something in env (env.premountspell) and then have Mount:GetCastActions() compose it into a macro.

There's almost no chance this should be the default because half of everyone is going to go apeshit. So it will likely remain something you have to edit into the action list.

There is the question of "removing" it again on dismount, which is an even bigger can of worms. That's probably not even a good idea (do you want to cancelaura Slow Fall and die), but I guarantee someone is going to complain about it.

The other option is

PreMountScript ...

and

PostMountScript

but then it can't test if you can actually cast any of those spells, so will probably fail.

This is in 9.1.1beta1 and onwards as action PreCast. Todo: add doco

Documentation added, considering this done.