Hi, Ken --
The rainbows just may surprise you. Their demise may not be quite as imminent as your fear. My hunch is that there may be some cooler, and still well-oxygenated water they can escape to during the heat of the day. I don't know how deep your pond is, but perhaps there is already a thermocline layer forming that will have adequate temp. and O2, for a while at least.
My experience with rainbows is that when they do expire (mostly from catch-and-release efforts gone awry, in my pond), they don't tend to become "floaters." In fact, all the dead trout I've found, have been on the bottom (I have a very clear pond). If I can't remove them (and offer them up to the feral "barn yard" cats around here) because they're too deep, the big channel cats in my pond do clean- up duty.
Anyway, I'm really glad you had a good experience with the rainbows ... but a few of them may fool you and linger on into early summer. -- Mike