Actually, as a developer of audio playback and editing software, i can tell you that this is definitely possible.
the only real limiting factor is the duration of the mp3 file you are wanting to loop.
when a piece of software chooses to play audio from an mp3 file (or any other file) there are several approaches that can be taken - you either send the audio to the sound card in it's compressed state (the easiest way), or you decompress it into PCM samples in RAM, and then send those samples to the sound card. when you send audio to a sound card in an uncompressed format, regardless of what operating system you are using, you have the option of specifying a sample to start and end a loop on.
so to answer your question - no I can not tell you what off the shelf packages can do this, as i have not investigated. however i do know that adobe audition (formally cool edit pro) by default reads mp3 files into ram and decompresses them before you even get a chance to play or edit these files, and yes, it can loop.
if you can be more specific about your requirements, (ie what o/s) and what exactly you want to do, i can research it more fully and let you know what software is available off the shelf to do this.