A while ago, an instructor (with a "Let's Try It" attitude) and I tried to some techniques while sitting in a chair.
First of all, you have to be aware of your surroundings. For example, many guests on the Jerry Springer show are caught unaware while sitting in chair even though they are in a highly emotional situation.
We used a wooden chair with arm rests. Generally speaking, you have to move forward and meet the incoming attack so you can pivot and/or blend.
After avoiding the inital attack, generally you have to stand up and out of the chair. In our experiments, the uke would fall away or jump over the chair. But I suspect in real life the attacker would collide with the back rest and take the chair with them (and anyone still sitting in the chair).
One last suggestion: put wood slats underneath the legs of the chair so you can practice on the mat without damaging it. If the chair is facing towards the shomen wall, the wood should be perpendicular to the shomen wall (ie one front leg & one back leg on one wood slat).