I've been thinking about this too over the last few days.
I currently have a bartop, but I intend to gut it soon to make an upright.
Thing is, I have this 'tradition' where I bring my bartop to work every year for the Christmas party (just got back from it tonight actually).
It's always a hit, so I still want something to bring along, and also for barbecues etc.
Obviously, a full size upright will not exactly lend itself to this, but I'm thinking that a box with sticks, buttons and a raspberry pi should do the trick.
I can either bring along a separate monitor, or hook it up to a TV if there is one available where I will be.
I'm thinking of building a similar thing to what is referred to above as a 'barstick', but I have seen them mostly referred to as a 'supergun', so it might be a good idea to use that term too when you are doing your research.
I'm glad I saw this thread because I hadn't seen them referred to as barsticks before.