There wouldn't really be any use for it. The only crusaders you should be leveling up in a formation would be the DPS and, if you're tanking, your frontrunners. That's easy to do manually. If you want to level up everyone in your formation, the "Level All" button works fine.

What scenario would merit needing to ONLY level formation crusaders?

You don't need all the characters around the Bush Whacker to be leveled, only the one that deals the DPS. If you're using more than one DPS crusader for damage, you should switch the second DPSer for a support crusader. The ABSOLUTE MOST a second DPSer would do would be buffing your overall damage by 100%, assuming the two DPSers have identical damage. If one is even slightly stronger than the other, your DPS is getting less than double from the weaker one. Support crusaders typically give FAR more buffs than just 100%, so an extra support crusader would be magnitudes better than two DPSers. So the only time when leveling more than one crusader near BW would make any sense would be the one scenario you never want to be in.

There's only one crusader I can think of off the top of my head where it's level actually matters even if it's not the tank/DPS, because one of that crusader's abilities gets buffed more and more as their level rises. Aside from them, your DPSer and tank should be the only crusaders getting leveled until the end of an idol run. Even then, rule of thumb is that if you're TANKING in an IDOL run, you're taking too long. Typically, you want to reset as soon as (or at least very soon after) you stop insta-killing monsters. More/weaker resets amounts to far more idols than fewer/stronger resets. This is especially true in early-game.

I am not a game designer or moderator for this game. Just another player. But I'm more than confident that I know the game well enough to offer constructive feedback about it. Teschio is another user that frequents these faqs that knows the game pretty well. He and I are not officials, but we're knowledgeable enough for our words to hold some form of weight. Obviously, we're not always right. Nobody is. But for this? I'm certain that it would be wasted work for the developers to create such a hotkey with nearly no use. They have more important things to deal with on their schedule.

EDIT: P.S. For objective runs where you might need/want to level up more than your DPSer/tank (e.g., an objective that constantly hurts all crusaders, so the levels/health keep them around longer), there are two things to note: 1) Again, typically, the "level all" button is enough to level all active crusaders enough to give them endurance. It's not PERFECTLY gold-efficient, but that rarely matters in such objectives. 2) If you NEED to be hyper-vigilant and absurdly-efficient with your levels to complete the objective, then your time would be better-spent farming for idols and generally getting stronger so that those objectives won't be so demanding anymore. Remember, this is an IDLE game. A long, time-consuming grind for power is the entire point of the game. You're not MEANT to be able to sweep everything in the game on your first try without massive amounts of time spent grinding for various ways to buff your strength. If objectives are giving you THAT much trouble, then you're taking them on too early.

I completely agree with Animenut. Tanking is never the way to go, unless you're trying an objective. And with very, VERY few exceptions (objectives that give bonus idols or increase the maximum numbers), you should not try an objective unless you can beat it quickly and easily, so again tanking becomes useless. It's a very niche tactic that should only be used a very limited number of times. Which is a shame, I feel there is this entire mechanic (healing and tanking) that nobody really uses 99.9% of the time.

If not doing one of the important objectives above, you really should not push a run: the second your progress slows down, it's time to reset. If you can't beat an area without resorting to tanking and healing, or waiting for XPs, reset immediately. Actually, reset the moment monsters start touching your formation, you will save a lot of time and therefore you will gain a lot more idols.

Patience is the name of the game, just farm some more, eventually almost all objectives will become trivial. Hell, I still have some to do because I can't be bothered to do them (they would be very easy, but the time spent on those is time I don't get more idols), and I'm at 6 trillion idols...

Thank you both.