Accessing hydraulic lines & resevoirs is perfectly straightforward in plenty of aircraft -- the C-130 as one example. With crew chiefs & their tools & spare parts on board, a temp repair on a hydraulic line would be possible. Highly unusual, but possible. Spare fluid is common enough to have on-board to replenish the resevoirs.
However, access to the tires... Nope. I can't think of anything where you could do that, and it would have to be a designed-in capability. Typically, crew-accessible areas are pressurized, but where the wheels retract, aren't. You have things like access to unpressurized bomb bays, and in unpressurized aircraft that's not a barrier, but still... If there are designs where the crew can access the tires in flight, I'm not picturing them.
As noted elsewhere, landing with a flat tire isn't typically a big deal.