Pottermore doesn't have any groundbreaking info on Quidditch at this time, but rather just a basic overview of what the game is all about.
Personally I deduce from canon that Quidditch is a down and dirty, full-blown contact sport where cheating abounds! Particularly when Gryffindor and Slytherin are playing against each other (well, okay, maybe Slytherin cheats a tad more liberally than Gryffindor. Ambition isn't always pretty.). There aren't many examples of magic used in Quidditch in canon.
In Prisoner of Azkaban, Harry, thinking he sees three Dementors, performs the Patronus charm during the Gryffindor/Ravenclaw game; however, it turns out to just be Malfoy, Flint, Crabbe, and Goyle dressed up as Dementors, trying to scare Harry and cause him to faint and fall. Madam Hooch did not call a foul on this against Harry. Professor McGonagall took care of business, though:
‘An unworthy trick!’ [Professor McGonagall] was shouting. ‘A low and cowardly attempt to sabotage the Gryffindor Seeker! Detention for all of you, and fifty points from Slytherin! I shall be speaking to Professor Dumbledore about this, make no mistake! Ah, here he comes now!’
Prisoner of Azkaban - page 195 - Bloomsbury - chapter 13, Gryffindor Versus Ravenclaw
The above, I believe, technically constitutes spectator interference; however, as Gryffindor was playing Ravenclaw that day, and not Slytherin, it wouldn't make sense to disqualify the Ravenclaw team for a ridiculous stunt Malfoy was responsible for.
That said, I can absolutely see disqualifying the Slytherin team in Order of the Phoenix for the Weasley is our King antics because Slytherin's Keeper, Bletchley, and the Slytherin spectators were participating in it. But Madam Hooch apparently
was passed out or something thought otherwise.
You already mentioned the Impervius charm that keeps Harry's glasses clear so he can see (and yet I am mentioned it again ... )
Perhaps it might be possible to cast the Meteolojinx Recanto spell to stop rain:
‘Yes, a lot of offices have been raining lately,’ said Mr Weasley. ‘Did you try meteolojinx recanto? It worked for Bletchley.’
‘Meteolojinx recanto?’ whispered Ron. ‘No, I didn’t. Thanks, D— I mean, thanks, Arthur.’
Deathly Hallows - page 210 - Bloomsbury - chapter 13, The Muggle-Born Registration Commission
It's unclear if Meteolojinx Recanto is just for indoor rain, or if it works only in small spaces. I think deliberately changing the weather to benefit a game of Quidditch would be considered weak and unsportsmanlike. Rain, mud, and sports are natural allies :)
In QTTA (pages 19-20), a Daily Profit article discusses how baskets attached to the hoops were disallowed, as it was impossible to standardize the size of the baskets. Why magical witches and wizards couldn't standardize baskets, I don't know.
Perhaps purposeful failure to knock nearby Bludgers away would constitute a foul if the Beaters' deliberate negligence caused one of their teammates to be injured. (QTTA pages 22-23)
Okay, as far as a good rule of thumb goes, I would imagine that magic used in Quidditch would have to be a spell or charm that does not give the receiver an unfair advantage over any other player, and that every individual playing had access to, such as Cushioning Charms and Braking Charms. For example, say all the members of the Hufflepuff and Gryffindor Quidditch teams wore glasses and couldn't see due to the rain. If they all got the Impervius Charm, fine. No problem. But if only Harry received the charm, that would be inequitable.
I looked at all the Quidditch scenes in the books and I actually didn't find an example, other than the Patronus Charm, of magic being used by one of the players in the game.
If the exhibition of Muggle duelling counts, then Harry, George, and Fred received a lifetime ban from Quidditch by Umbridge after they beat up Draco Malfoy following the Gryffindor/Slytherin game in Order of the Phoenix.