The French did develop such a weapon in 1940, and it did achieve noticeable results, but of course by then there wasn't enough time to produce and deploy enough of them to turn the tide. I'm talking about the Laffly W15 TCC, which was essentially a Laffly armored troop transport with a rear-mounted 47 mm gun. While lightly armored, it made up for it with good mobility, and proved quite suited to hit-and-run defensive tactics against tank formations.
Different design from what I expected, no troop transport option, very little armor, but for the correct strategy. The rear mounted gun is actually useful in combination with a towed second gun, as both are pointing in the same direction, making it easy to quickly drive away. Cool thing.