Yes, bootmii is removable, and its likely the best you're going to get, seeing some mentioned its running 4.0. All the other dumpers i know of either require a trucha capable IOS, which isn't present on 4.0, or cIOS.
Also, bootmii gives you the Wii's keys, which we'd need anyway to decrypt the dumps.
You could bring YAWND to try first, but it likely won't work because the signing bug is probably fixed.
Any way you look at it, you'll likely need something installed to grab the dump. If you use bootmii, uninstalling afterwards covers it pretty well, unless they explicitly go looking for it. It'd leave even less if installed as boot2.
Just be sure to bring an NUS Downloader wad of the "official" IOS254, so you can restore it if you install bootmii as IOS. I *think* unaltered NUSD wads are properly signed, so it should install fine.