If the data is in fact already UTF-8 encoded, then the _utf8_on() function in the Encode module would be what you're looking for. It sets the interal UTF-8 flag on a string, but does not otherwise mangle it.
That got it, thanks. I know I tried this last night (read: at 0500 this morning) but I got unexpected results and backed it out. I can only assume I typo'd or did something else dumb at the time. Thanks also to liz for the header idea -- I was pretty sure I already had it that way, but I checked to be sure.