We have a handbag highchair and I used it a few times when ds was small, I agree with cod, we had to have him away from the table as it was usually about eye level, it was then difficult for him to feed himself. They are great if you cant face packing a bulky seat. I still have mine if you want to try it.
get a harness, they all come with the attaxhment bits which you can attach to most chairs.
but, quite seriously, where in the world are you going that you need to take one with you? i think i have travelled a quite a bit with a baby in tow, and dont think have ever been in aplace where couldnt find a high chair.
Have you tried Totseat? It's pretty good, all adjustable and fits on any chair. Maddy prefers it to her normal highchair! I got mine from their website totseat.com, but I believe you can get them at John Lewis.