Pump head suitable for for small tires
It sounds like the issue that you're having is very similar to the one that time-triallists and triathletes have with disc wheels. Luckily they've come up with a solution for exactly that. What you're looking for is called a disc wheel valve adaptor, often referred to as a "crack pipe" for obvious reason due to its appearance. The most widely used and durable one is the silca version (https://silca.cc/products/disc-wheel-mini-chuck) because of the replaceable seals and tiny head which is easy to fit in the tiny space with the valve. There are plenty of options out there for less money as well such as this one by Fast Forward: 
http://www.wigglestatic.com/product-media/5360051984/ffwd-valve-adapter.jpg?w=430&amp;h=430&amp;a=7