My god OP, you described my previous experience with IEMs and my last band, to a very large effing T.
We ran our own sound, and every one had control of their IEM mix via the iphone app, and my one singer would STILL look over at me and say, "I can't hear you" sometimes. NOT MATTER HOW MANY TIMES I TOLD HER THAT SHE HAS CONTROL OF THAT, AND I DO NOT.
I cannot overstate how frustrating that was.
Anyway, a couple of thoughts, some have been mentioned:
1. There are some places or soundmen we use where, I'll plug directly into their powered wedge, BEFORE they even notice or before we even talk about it, and when I tell them what I'm doing, after the fact, they pause, but let it go. In fact, I usually tell them that nothing is changed, they can just take that same monitor line, and run it to the IEM instead of the wedge, and that is the ONLY difference. That can be done at the monitor send area, or at the wedge area, their choice. I even had a 30' XLR cable of my own, that I used to bring, for such instances.
That may not be 100% for you, but maybe that could be plan A.
2. It depends on what kind of sound you're going for, but I have what I call my "casino" preset, which is the OCD and EP booster as the main dirt tones, with a couple of effects, to run into the front of a combo/clean amps, and that works EXTREMELY well for me.
3. The small power amp idea would work too, you would just have to make sure you can get to the 1/4" cable that goes to the speaker, those are usually wired in pretty tight with combos, and it's almost impossible to just put a power amp on top of a combo, there's usually not enough slack.
4. You could just set a preset for use into the effects return of backline amps, but there is one casino I play that uses Roland Blues cubes, that DON'T have effects loops, so, see #2.