Why so many people are assuming we need a mod for this? From my understanding, we can just not allow Baton Pass alongside a boosting move on the teambuilder the same way we did with Mega Gengar and Hypnosis. Shadow Tag wouldn't just fail if Mega Gengar came on a Pokémon that's already sleeping, the same way Baton Pass shouldn't if your opponent used Swagger (which is 100% their fault if they did that lol). Contrary to Dynamax, I won't go to great lenghts to support the clause over the simple ban as the dimension of it is much lower, but if do the clause, I don't see why a mod is necessary over a teambuilder limit.
To be completely honest, I thought BP clause was that, initially, given BP clause, in historical context, is... well, that, where you are not allowed to run BP and anything that boosts your own stats (like SD / Speed Boost etc.) so I got a bit surprised reading the whole thread that the BP clause was an in-battle mod.
However, upon thinking about it, I realized that the clause this way does not prevent (at least on first look) passing boosts completely, as someone may Dynamax, get some boosts (like from Max Knuckle or Airstream) and BP them anyways. Would this be worth handling? I can see the disadvantages of such strategy, because you can't BP while Dynamaxed (and trying to Max Guard to stall means less boost to get) and the recipient can be copied by Ditto or countered in some other way and thus forcing a switch and potentially wasting the opponent's Dynamax.
I can only theorymon about whenever BPing Dynamax boosts would be broken, unless we see how it goes. If it does end up being broken, either BP clause has to make it so BP users cannot Dynamax or BP should just be banned. Though, again, I have some doubts it's gonna be a game breaker, so for now, I am not gonna dive into it too much.
All in all, my personal stance is that BP clause should probs be "prevent running BP with self-boosting things", if that is possible and see if passing Dynamax boosts is broken. I do understand that Ubers Premier League is a thing right now and maybe "testing" wouldn't be a very appropriate decision in terms of affecting the tournament, so
maybe ban BP initially and give a try to a BP clause after UPL? An initial ban on BP is "safer" because we know it's gonna address the problem with BP completely, while the BP clause on teambuilder level can potentially fail, if we do not artificially restrict Dynamaxing on BP users. But ultimately, I'd like to at least try BP clause at some point (on teambuilder level), because banning BP is already a complex ban (although the less complex one) and I personally think it's fine for Ubers, being the last official tier, to enact such clauses, in order to preserve an element's "official" presence on Smogon tiers.