With a portable recorder like the Zoom (do they still make those, I'll have to look that up)or a smart phone, I'd have to transfer the vocal melodies to BIAB via an SD card, right?
I use a Zoom H1N quite often. A big attraction is that it's small enough (just) to keep in a pocket. Whatever you use needs to be with you or it's useless! It connects via USB as a memory device, so it's just a matter of plugging it in and copy or move the files to you PC.
My other thought, like Jim's, is your mobile 'phone. The quality is likely not so good, but the chances of you having it with you are pretty high.