The Fireteam Order Sergeants with Multi Sniper and Heavy Rocket Launcher are both solid options. Obviously the Multi Sniper TO Specialist Sergeant is an essential MO touchstone unit. The Hospitaller HMG either in the Hospitaller or Magister Fireteam works well. It is only OK outside the Fireteam due to the issues with Frenzy on long range fire support units. Matching that is the Magister Knight Missile Launcher. Low burst is the killer here, but it is, at least, long range and the template is obviously attractive (just like the HRL in the Order Sergeants link). I think that the Crusader Knight HMG has been hastily dismissed. Don't think about dropping it close, think about it as a flexible second reserve piece that you place in the best position after you see what the enemy is doing. Now the Knight of the Holy Sepulchre with Spitfire deserves a mention here because Holo 2 can get it across a reasonable gap into cover to avoid extreme range AROs. This isn't ideal, but it can work on most tables. So, that's not a long list, and a couple of the options are pretty marginal in this role. But it's not nothing, and I think it's more than most people realise.
I hadn't really considered the linked Magister ML, it's not a terrible option. It's got a 50% chance of knocking out a linked alguacile ML with a 30% chance of going unconcious himself, but only a 15% chance of dying outright, which means that you'd probably be able to doctor him and get another shot. I think the most interesting option that's come up so far to me is the spitfire peacemaker. I've always opted for the heavy shotgun but if you're going first it's a really strong option, you can reserve the model and place it last somewhere within optimal range of the ARO piece you want dead. It's got a repeater so you can drop MML2 on it from anywhere and then go to town. It's vulnerable to hackers and especially in MO with no KHD that can be a problem. Also if you're going 2nd it might be hard to keep it from just getting killed by a cheap warband, but it's got some defenses in that regard at least with the auxbot to help protect itself. All in all I'm impressed with it in paper and it's definitely something I intend to try out at some point when I don't want to bring the hospitaller HMG.
Peacemaker Spitfire is a good call. I often forget that the combat remotes are there as an option for Military Orders, but they remain strong choices.