This seems like a worthwhile action for any of us who can afford it. Actually it's not just about the money -- if a dojo is run in the spirit of "give what you can", and we're honest about what we can give (in the form of work, goods, cash, help with classes, our spirit and presence on the mat), a lot will come of it.
I've posted pictures of our dojo, most recently the "before" and "after" photos. All that work was done by our senseis and students. Privately, I think of our garden as the "stone soup" garden. The difference between the "stone soup" story and our garden is that rather than being tricked into giving, the people of our dojo understood what they were doing as they contributed compost, seeds, plants, soil, gravel, rocks, money, sweat, blisters and imagination...a bit from here, a bit from there, and the whole is more than the sum of its parts. Done with the right spirit, it'll all work out.
Who owns the dojo.
Do you realize that by volunteering your time, materials and money to fix up the dojo you are increasing the value of the property. In fact you are making the owner of the dojo wealthier.
That is ok as long as you realize that the owner at some time in the future might want to sell the property.
I had a similar experience at a karate studio.