Yes! Absolutely yes. In Ears are for everyone. Including horn players.
Custom in-ear monitors are for any musician who wants to protect their hearing and have more control over their mix. And that includes trumpet players, sax players, and even tuba players! (There are never enough posts about tuba players but that’s a different story…)
Granted, we don’t see as many horn players ordering ears as say vocalists, drummers, or guitarists— but that’s just a cultural phenomena, not a technological hurdle. Of course, there's the concern that blowing into an instrument will alter the position of the tympanic membrane and alter the pressure in the middle ear cavity, but since we require open mouth impressions for our in-ears, blowing does not compromise the seal.
I’ve always suggested that wind players opt in for the additional ambient ports but I’ve talked with many players who went for the fully sealed units and were just as happy. Still, for the extra $50, I’d include it for peace of mind. Since the goal of the port is to let stage noise in, by default it also compromises the seal which eliminates any lingering fear of pressure build up.
If you’ve been on the fence about the transition, start by using universal fit in-ears before committing to the customs. If it feels too foreign and after a few sound-checks, you don’t think that it’s something you’ll ever get used to, then no harm, no foul. But if you start to notice just how tight you're playing gets and just how god you sound, then make the leap and help us spread the good word amongst your fellow horn sections.
Like I said at the beginning of this post, in-ears are for everyone. I hate to see horn players missing the boat.