I am a musician spending a few months away my home. I fear that I may lose a good chunk of skill if I am unable to practice my instrument during this time. It is completely unfeasible for me to buy an instrument while I'm away for both space and cost reasons (I play piano). Does anyone know of any sort of group/club/system (generically) that revolves around communal instruments? Perhaps something like a group where you pay some nominal fee every month for access to instruments. I won't specify where I am as I'd like to keep this answer as generic as possible.
A person who plays the piano has difficulties in practicing. Like you questioned skill can go down without practice. Piano is a larger piece of musical instrument that is not portable easily. A key board is a solution. In this instance the cost is a deciding factor for him.
Taking all these considerations there can only be one solution. Are you in a location where there are clubs or hotels that have a piano. You could volunteer to play a few pieces everyday to entertain the club members or the guests. As a starter offer it free. Ask Them if they would approve a hour or two earlier than the regular time to help you to be in your best performance. This should help you with your skill and practice.
One other idea is to volunteer to teach children music in a school where there is a piano or keyboard after school hours.
You could also think of a home where there is a piano and children or adults need piano lessons. This way you could help them and in turn help your self with maintaining your skills and practice sessions.
I hope you are in a country or a city where clubs, homes, hotels and schools are nearby.
Taking a keyboard (from £20 second hand in U.K.) is an obvious answer, but probably not viable to you as a pianist - the action will not do the job, although the black and white bits are in the right place ! I got over the same problem in a different way. Learn another instrument. I take a flute or clarinet away when there is little luggage room. They can be played without upsetting anyone. Your musical knowledge will be enhanced by applying it to a different means of making sounds.This will not necessarily help you as a piano player, but as a musician it will be good. You could (to add to existing answer) also source music shops, and find quiet times there. Talking to the guys in the shops will probably reveal other options.